{
  "site": "Sudoku Coach",
  "url": "https://sudoku.franklab.us",
  "description": "Sudoku Coach teaches techniques step by step, from singles to X-Wing and beyond.",
  "techniques": [
    {
      "title": "Single Candidate in Sudoku: How to Spot It",
      "slug": "single-candidate",
      "url": "https://sudoku.franklab.us/techniques/single-candidate/",
      "description": "Learn the Single Candidate Sudoku technique: check one cell's row, column, and box until only one legal number remains.",
      "level": "Beginner",
      "order": 1,
      "category": "single",
      "summary": "A Single Candidate occurs when a cell has only one possible number left after checking row, column, and box constraints.",
      "quickAnswer": "A Single Candidate in Sudoku is one empty cell with exactly one legal number left. Check the cell's row, column, and 3×3 box; if those three areas rule out eight digits, the remaining digit must be placed in that cell.",
      "prerequisites": [],
      "recognitionRules": [
        "Start with one empty cell that has many filled neighbors in its row, column, and 3×3 box.",
        "Check the row and cross off any digits already used there.",
        "Check the column and cross off any digits already used there.",
        "Check the 3×3 box and cross off any digits already used there.",
        "If exactly one digit from 1–9 is not crossed off, place that digit in the cell."
      ],
      "eliminationRule": "No other cells are changed; the target cell is solved because it has one legal option.",
      "practiceGoal": "Practice asking 'what can go in this cell?' and prove the answer by checking row, column, and box before placing a number.",
      "related": [
        "hidden-single",
        "candidate-notes",
        "naked-pair"
      ],
      "keywords": [
        "single candidate sudoku",
        "naked single sudoku",
        "beginner sudoku technique"
      ],
      "faqs": [
        {
          "question": "What is a Single Candidate in Sudoku?",
          "answer": "A Single Candidate is an empty cell with only one legal number left after its row, column, and 3×3 box are checked."
        },
        {
          "question": "Is Single Candidate the same as Naked Single?",
          "answer": "Yes. Many Sudoku guides call this technique a Naked Single because the cell's candidate list has only one visible option."
        },
        {
          "question": "When should beginners use Single Candidate?",
          "answer": "Use it early and often, especially on empty cells surrounded by many filled numbers. Those cells are more likely to have eight digits blocked already."
        },
        {
          "question": "Does Single Candidate require candidate notes?",
          "answer": "Candidate notes help, but they are not required. You can also test the digits 1 through 9 manually against the cell's row, column, and box."
        }
      ]
    },
    {
      "title": "Hidden Single in Sudoku: How to Spot It",
      "slug": "hidden-single",
      "url": "https://sudoku.franklab.us/techniques/hidden-single/",
      "description": "Learn the Hidden Single Sudoku technique: when a number has only one possible position in a row, column, or box.",
      "level": "Beginner",
      "order": 2,
      "category": "single",
      "summary": "A Hidden Single occurs when a number has only one possible position within a row, column, or box.",
      "quickAnswer": "A Hidden Single in Sudoku is when a number can go in only one cell within a row, column, or box. The cell may have several candidates, but that number has only one possible position in the unit, so it must be placed there.",
      "prerequisites": [
        "single-candidate"
      ],
      "recognitionRules": [
        "Choose one row, column, or box that has several empty cells.",
        "Pick a missing number from that unit.",
        "Check every empty cell where that number could go.",
        "If there is only one possible cell for that number, place it there."
      ],
      "eliminationRule": "No external elimination is needed; the number is placed because it has one legal position in the unit.",
      "practiceGoal": "Learn to scan by number within a unit, not only by candidate count in each cell.",
      "related": [
        "single-candidate",
        "candidate-notes",
        "hidden-pair"
      ],
      "keywords": [
        "hidden single sudoku",
        "single position sudoku",
        "beginner sudoku solving"
      ],
      "faqs": [
        {
          "question": "What is a Hidden Single in Sudoku?",
          "answer": "A Hidden Single is a number that has only one possible position in a row, column, or box."
        },
        {
          "question": "How is Hidden Single different from Single Candidate?",
          "answer": "Single Candidate focuses on one cell with one candidate. Hidden Single focuses on one number with one possible position in a unit."
        },
        {
          "question": "Why is it called hidden?",
          "answer": "It is hidden because the target cell may still have multiple candidates, even though one number is forced there."
        },
        {
          "question": "Is Hidden Single beginner-friendly?",
          "answer": "Yes. It is one of the most important beginner techniques after Single Candidate."
        }
      ]
    },
    {
      "title": "Candidate Notes in Sudoku: How to Use Them",
      "slug": "candidate-notes",
      "url": "https://sudoku.franklab.us/techniques/candidate-notes/",
      "description": "Learn how to write and maintain Sudoku candidate notes so every later technique has reliable evidence.",
      "level": "Beginner",
      "order": 3,
      "category": "candidate-management",
      "summary": "Candidate notes are the small possible numbers in unsolved cells; they show which digits remain legal after checking the row, column, and box.",
      "quickAnswer": "Candidate notes in Sudoku are small numbers written in empty cells to track every digit that could still legally fit. Good notes include only numbers not already blocked by the cell's row, column, or 3×3 box, then get updated after each placement or elimination.",
      "prerequisites": [
        "single-candidate"
      ],
      "recognitionRules": [
        "Choose an unsolved cell and inspect its row, column, and 3×3 box.",
        "Remove every number already present in those three areas.",
        "Write only the remaining legal digits as candidates for that cell.",
        "Update affected notes whenever a number is placed nearby."
      ],
      "eliminationRule": "Candidate notes do not eliminate by themselves; they record legal options so singles, pairs, locked candidates, and advanced patterns can be found safely.",
      "practiceGoal": "Build a habit of accurate notes before using candidate-based techniques.",
      "related": [
        "single-candidate",
        "hidden-single",
        "locked-candidates"
      ],
      "keywords": [
        "sudoku candidate notes",
        "sudoku pencil marks",
        "how to write candidates in sudoku"
      ],
      "faqs": [
        {
          "question": "What are candidate notes in Sudoku?",
          "answer": "Candidate notes, also called pencil marks, are the possible numbers that can still go in an empty cell."
        },
        {
          "question": "When should I start using candidate notes?",
          "answer": "Start using them when scanning no longer reveals easy singles or when you want to practice candidate-based techniques accurately."
        },
        {
          "question": "Should candidate notes include every possible number?",
          "answer": "They should include every legal possibility for the cell, but not numbers already blocked by the row, column, or box."
        },
        {
          "question": "Do bad notes cause wrong Sudoku moves?",
          "answer": "Yes. Missing or extra candidates can hide singles, create false pairs, or make eliminations look valid when they are not."
        }
      ]
    },
    {
      "title": "Locked Candidates in Sudoku: How to Spot Them",
      "slug": "locked-candidates",
      "url": "https://sudoku.franklab.us/techniques/locked-candidates/",
      "description": "Learn the Locked Candidates Sudoku technique: when candidates are trapped in one line or one box and can be removed elsewhere.",
      "level": "Intermediate",
      "order": 4,
      "category": "candidate-elimination",
      "summary": "Locked Candidates occur when all possible positions for a digit in a box lie on one row or column, or all positions in a row or column lie inside one box.",
      "quickAnswer": "Locked Candidates in Sudoku happen when a candidate digit is confined to one row or column inside a 3×3 box, or confined to one box inside a row or column. Because the digit must be placed in that shared area, the same digit can be removed from the rest of the affected row, column, or box.",
      "prerequisites": [
        "candidate-notes",
        "hidden-single"
      ],
      "recognitionRules": [
        "Choose one digit and inspect its candidate positions in a single box, row, or column.",
        "Confirm all candidates for that digit are locked into the same intersecting line or box.",
        "Identify the cells outside the locked area that share the same row, column, or box.",
        "Remove only that locked digit from those outside cells."
      ],
      "eliminationRule": "If a digit is locked to one row or column within a box, remove that digit from the rest of that row or column outside the box. If a digit in a row or column is locked inside one box, remove it from the rest of that box.",
      "practiceGoal": "Learn to track one digit across an intersecting box and line before removing candidates.",
      "related": [
        "candidate-notes",
        "naked-pair",
        "hidden-pair"
      ],
      "keywords": [
        "locked candidates sudoku",
        "pointing pairs sudoku",
        "claiming candidates sudoku"
      ],
      "faqs": [
        {
          "question": "What are Locked Candidates in Sudoku?",
          "answer": "Locked Candidates are candidate digits that are confined to the overlap of a row or column and a 3×3 box, allowing safe eliminations outside that overlap."
        },
        {
          "question": "Are Locked Candidates the same as pointing pairs?",
          "answer": "Pointing pairs and triples are one common form of Locked Candidates, where candidates inside a box point along one row or column."
        },
        {
          "question": "What is the difference between pointing and claiming?",
          "answer": "Pointing starts from a box and removes from a line. Claiming starts from a row or column and removes from the rest of the box."
        },
        {
          "question": "Do Locked Candidates require candidate notes?",
          "answer": "Yes. You need accurate candidate notes to prove the digit is truly locked before eliminating it elsewhere."
        }
      ]
    },
    {
      "title": "Naked Pair in Sudoku: How to Spot It",
      "slug": "naked-pair",
      "url": "https://sudoku.franklab.us/techniques/naked-pair/",
      "description": "Learn how Naked Pairs work in Sudoku, when to look for them, and how to remove candidates safely.",
      "level": "Intermediate",
      "order": 5,
      "category": "candidate-elimination",
      "summary": "A Naked Pair occurs when two cells in the same row, column, or box contain exactly the same two candidates.",
      "quickAnswer": "A Naked Pair in Sudoku is a pattern where two cells in the same unit contain exactly the same two candidates. Those two numbers must occupy those two cells, so the same candidates can be removed from other cells in that row, column, or box.",
      "prerequisites": [
        "single-candidate",
        "hidden-single",
        "candidate-notes"
      ],
      "recognitionRules": [
        "Choose one row, column, or box and look only inside that unit.",
        "Find two cells that contain the exact same two candidates.",
        "Confirm both cells are in the same unit and contain no extra candidates.",
        "Remove those two candidates from every other cell in that unit."
      ],
      "eliminationRule": "Remove the paired candidates from other cells in the same row, column, or box, but not from the pair itself.",
      "practiceGoal": "Recognize the two matching candidate cells first; the eliminations come after the pattern is confirmed.",
      "related": [
        "locked-candidates",
        "hidden-pair",
        "x-wing"
      ],
      "keywords": [
        "naked pair sudoku",
        "sudoku candidates",
        "sudoku technique practice"
      ],
      "faqs": [
        {
          "question": "What is a Naked Pair in Sudoku?",
          "answer": "A Naked Pair is two cells in one row, column, or box that contain the exact same two candidates."
        },
        {
          "question": "How do you spot a Naked Pair?",
          "answer": "Look for two cells in the same unit with identical two-candidate lists, such as {2, 7} and {2, 7}."
        },
        {
          "question": "What can you eliminate with a Naked Pair?",
          "answer": "You can remove the two paired candidates from every other cell in that same row, column, or box."
        },
        {
          "question": "What is the difference between Naked Pair and Hidden Pair?",
          "answer": "A Naked Pair is visible because the two cells contain only the pair. A Hidden Pair is hidden among extra candidates and must be uncovered by looking at where two numbers can go."
        }
      ]
    },
    {
      "title": "Hidden Pair in Sudoku: How to Spot It",
      "slug": "hidden-pair",
      "url": "https://sudoku.franklab.us/techniques/hidden-pair/",
      "description": "Learn how Hidden Pairs work in Sudoku, why they are harder to see than Naked Pairs, and how to clean up candidates.",
      "level": "Intermediate",
      "order": 6,
      "category": "candidate-elimination",
      "summary": "A Hidden Pair occurs when two numbers can only appear in the same two cells within a unit, even if those cells also show other candidates.",
      "quickAnswer": "A Hidden Pair in Sudoku is when two candidates appear only in the same two cells of a row, column, or box. Those two cells must contain those two numbers, so all other candidates can be removed from those cells.",
      "prerequisites": [
        "single-candidate",
        "hidden-single",
        "candidate-notes",
        "naked-pair"
      ],
      "recognitionRules": [
        "Choose one row, column, or box and scan by number positions.",
        "Pick two numbers and check where each can appear.",
        "If both numbers are limited to the same two cells, they form a Hidden Pair.",
        "Remove all other candidates from those two cells."
      ],
      "eliminationRule": "Remove extra candidates from the two Hidden Pair cells; do not remove the pair numbers from those cells.",
      "practiceGoal": "Train yourself to scan by number positions, not just by visible two-candidate cells.",
      "related": [
        "locked-candidates",
        "naked-pair",
        "hidden-single"
      ],
      "keywords": [
        "hidden pair sudoku",
        "sudoku pair technique",
        "sudoku candidates"
      ],
      "faqs": [
        {
          "question": "What is a Hidden Pair in Sudoku?",
          "answer": "A Hidden Pair is two numbers that can only go in the same two cells within a row, column, or box."
        },
        {
          "question": "How is Hidden Pair different from Naked Pair?",
          "answer": "A Naked Pair is visible as two identical two-candidate cells. A Hidden Pair may be inside cells with extra candidates, so you find it by tracking positions for two numbers."
        },
        {
          "question": "What do you eliminate with a Hidden Pair?",
          "answer": "You remove all other candidates from the two pair cells."
        },
        {
          "question": "Is Hidden Pair harder than Naked Pair?",
          "answer": "Usually yes, because the pattern is hidden among extra candidates instead of appearing as two clean two-candidate cells."
        }
      ]
    },
    {
      "title": "Naked Triple in Sudoku: How to Spot It",
      "slug": "naked-triple",
      "url": "https://sudoku.franklab.us/techniques/naked-triple/",
      "description": "Learn the Naked Triple Sudoku technique: three cells in one unit reserve three candidates, allowing eliminations from the rest of that unit.",
      "level": "Advanced",
      "order": 7,
      "category": "candidate-elimination",
      "summary": "A Naked Triple occurs when three cells in a row, column, or box contain only the same three candidates between them.",
      "quickAnswer": "A Naked Triple in Sudoku is a set of three cells in one unit whose candidates are limited to the same three numbers. Those three numbers must fill those cells, so you can remove them from every other cell in that unit.",
      "prerequisites": [
        "naked-pair",
        "candidate-notes"
      ],
      "recognitionRules": [
        "Pick one row, column, or box with several candidate notes.",
        "Look for three unsolved cells whose notes use only the same three numbers, such as 1, 5, and 9.",
        "Confirm no cell in the triple contains a fourth candidate.",
        "Remove those three numbers from every other cell in that same unit."
      ],
      "eliminationRule": "The triple's three numbers are reserved for the three triple cells, so those numbers can be removed from other cells in the unit.",
      "practiceGoal": "Train yourself to scan for three cells that behave like a larger Naked Pair before attempting harder patterns.",
      "related": [
        "naked-pair",
        "hidden-triple",
        "x-wing"
      ],
      "keywords": [
        "naked triple sudoku",
        "sudoku triples",
        "advanced sudoku technique"
      ],
      "faqs": [
        {
          "question": "What is a Naked Triple in Sudoku?",
          "answer": "A Naked Triple is three cells in one row, column, or box that together contain only three possible numbers."
        },
        {
          "question": "How is a Naked Triple different from a Naked Pair?",
          "answer": "A Naked Pair uses two cells and two numbers. A Naked Triple uses three cells and three numbers."
        },
        {
          "question": "Do the three cells need identical notes?",
          "answer": "No. The cells can show combinations like 15, 59, and 159 as long as the union is exactly three numbers."
        },
        {
          "question": "What should I do after finding a Naked Triple?",
          "answer": "Remove the triple's numbers from other cells in the same unit, then look for new singles or pairs."
        }
      ]
    },
    {
      "title": "Hidden Triple in Sudoku: How to Spot It",
      "slug": "hidden-triple",
      "url": "https://sudoku.franklab.us/techniques/hidden-triple/",
      "description": "Learn the Hidden Triple Sudoku technique: three numbers are limited to three cells, so extra candidates inside those cells can be removed.",
      "level": "Advanced",
      "order": 8,
      "category": "candidate-elimination",
      "summary": "A Hidden Triple occurs when three numbers can appear only in the same three cells inside one unit.",
      "quickAnswer": "A Hidden Triple in Sudoku is a set of three numbers that can only go in three cells of the same row, column, or box. Keep those three numbers in those cells and remove any extra candidates from the same cells.",
      "prerequisites": [
        "hidden-pair",
        "naked-triple"
      ],
      "recognitionRules": [
        "Choose one row, column, or box and focus on three target numbers.",
        "Mark every cell where those numbers can appear.",
        "Confirm the three numbers are limited to the same three cells.",
        "Remove all other candidates from those three cells."
      ],
      "eliminationRule": "Because the three numbers must occupy the three hidden-triple cells, unrelated candidates inside those cells can be removed.",
      "practiceGoal": "Practice scanning by number, not by cell, so hidden patterns become easier to see.",
      "related": [
        "hidden-pair",
        "naked-triple",
        "naked-pair"
      ],
      "keywords": [
        "hidden triple sudoku",
        "sudoku hidden triples",
        "advanced sudoku candidates"
      ],
      "faqs": [
        {
          "question": "What is a Hidden Triple in Sudoku?",
          "answer": "A Hidden Triple is three numbers that can only appear in three cells within one row, column, or box."
        },
        {
          "question": "What gets eliminated in a Hidden Triple?",
          "answer": "You remove extra candidates from the hidden-triple cells themselves, not from every other cell."
        },
        {
          "question": "Why is it called hidden?",
          "answer": "The triple is hidden because the cells may contain extra candidates that disguise the three-number pattern."
        },
        {
          "question": "Should beginners learn Hidden Triples early?",
          "answer": "No. Learn Hidden Singles, Naked Pairs, and Hidden Pairs first, then use Hidden Triples when easier scans stall."
        }
      ]
    },
    {
      "title": "X-Wing in Sudoku: How to Spot It",
      "slug": "x-wing",
      "url": "https://sudoku.franklab.us/techniques/x-wing/",
      "description": "Learn the X-Wing Sudoku technique with a simple candidate pattern, clear elimination rule, and practice-focused explanation.",
      "level": "Advanced",
      "order": 9,
      "category": "fish",
      "summary": "An X-Wing occurs when one candidate appears in exactly the same two columns across two rows, or the same two rows across two columns.",
      "quickAnswer": "An X-Wing in Sudoku is a rectangle pattern for one candidate. If that candidate appears in the same two columns in two different rows, it must occupy opposite corners of the rectangle, so that candidate can be removed from other cells in those columns.",
      "prerequisites": [
        "single-candidate",
        "hidden-single",
        "candidate-notes",
        "locked-candidates",
        "naked-pair"
      ],
      "recognitionRules": [
        "Choose one candidate number and ignore the other candidates for a moment.",
        "Find two rows where that number appears in exactly the same two columns, or two columns where it appears in exactly the same two rows.",
        "Confirm the four candidate positions form a rectangle.",
        "Remove that candidate from other cells in the affected columns or rows."
      ],
      "eliminationRule": "In a row-based X-Wing, remove the candidate from other cells in the two matching columns. In a column-based X-Wing, remove it from other cells in the two matching rows.",
      "practiceGoal": "Focus on one candidate at a time and look for matching two-position rows or columns.",
      "related": [
        "locked-candidates",
        "naked-pair",
        "hidden-pair"
      ],
      "keywords": [
        "x-wing sudoku",
        "advanced sudoku technique",
        "sudoku fish pattern"
      ],
      "faqs": [
        {
          "question": "What is an X-Wing in Sudoku?",
          "answer": "An X-Wing is a rectangle pattern where one candidate appears in two matching positions across two rows or two columns."
        },
        {
          "question": "How do you spot an X-Wing?",
          "answer": "Pick one candidate and find two rows with that candidate in the same two columns, or two columns with that candidate in the same two rows."
        },
        {
          "question": "What can you eliminate with X-Wing?",
          "answer": "You eliminate that candidate from the other cells in the affected columns or rows outside the rectangle."
        },
        {
          "question": "Is X-Wing an advanced Sudoku technique?",
          "answer": "Yes. X-Wing is usually considered advanced because it requires scanning candidate positions across multiple rows or columns."
        }
      ]
    },
    {
      "title": "XY-Wing in Sudoku: How to Spot It",
      "slug": "xy-wing",
      "url": "https://sudoku.franklab.us/techniques/xy-wing/",
      "description": "Learn the XY-Wing Sudoku technique: a pivot cell and two wing cells force a shared candidate elimination.",
      "level": "Advanced",
      "order": 10,
      "category": "chain",
      "summary": "An XY-Wing uses three bivalue cells: one pivot and two wings. The shared wing candidate can be removed from cells that see both wings.",
      "quickAnswer": "An XY-Wing in Sudoku uses a pivot cell with candidates X/Y and two wing cells X/Z and Y/Z. Any cell that sees both wings cannot contain Z.",
      "prerequisites": [
        "naked-pair",
        "x-wing"
      ],
      "recognitionRules": [
        "Find a bivalue pivot cell with two candidates, such as 1 and 2.",
        "Find one wing that sees the pivot and contains 1 plus a shared candidate, such as 1 and 7.",
        "Find another wing that sees the pivot and contains 2 plus the same shared candidate, such as 2 and 7.",
        "Remove the shared candidate from any cell that sees both wings."
      ],
      "eliminationRule": "Whichever value the pivot takes, one wing must become the shared candidate, so any cell seeing both wings cannot also be that candidate.",
      "practiceGoal": "Practice naming the pivot and wings before making the elimination.",
      "related": [
        "x-wing",
        "xy-chain",
        "naked-pair"
      ],
      "keywords": [
        "xy-wing sudoku",
        "sudoku xy wing",
        "sudoku wing technique"
      ],
      "faqs": [
        {
          "question": "What is an XY-Wing in Sudoku?",
          "answer": "An XY-Wing is a three-cell pattern with one pivot and two wings that eliminates a shared candidate from cells seeing both wings."
        },
        {
          "question": "Do all three XY-Wing cells need two candidates?",
          "answer": "Yes. The classic XY-Wing uses three bivalue cells."
        },
        {
          "question": "What is the pivot in an XY-Wing?",
          "answer": "The pivot is the bivalue cell that sees both wings and contains the two non-shared candidates."
        },
        {
          "question": "What candidate is removed?",
          "answer": "Remove the candidate shared by the two wings from any cell that sees both wing cells."
        }
      ]
    },
    {
      "title": "Swordfish in Sudoku: How to Spot It",
      "slug": "swordfish",
      "url": "https://sudoku.franklab.us/techniques/swordfish/",
      "description": "Learn the Swordfish Sudoku technique: three rows and three columns lock a candidate into a fish pattern.",
      "level": "Expert",
      "order": 11,
      "category": "fish",
      "summary": "A Swordfish occurs when one candidate is confined to the same three columns across three rows, or the same three rows across three columns.",
      "quickAnswer": "A Swordfish in Sudoku is a fish pattern where one candidate appears only in the same three columns across three rows, or the same three rows across three columns. You can remove that candidate from the rest of those columns or rows.",
      "prerequisites": [
        "x-wing",
        "naked-triple"
      ],
      "recognitionRules": [
        "Pick one candidate number and scan rows or columns only for that number.",
        "Find three rows where the candidate appears in no more than three matching columns.",
        "Confirm each selected row has its candidate positions only inside those three columns; do not include a row with an extra candidate outside the fish.",
        "Remove the candidate from other cells in those columns."
      ],
      "eliminationRule": "The candidate must occupy the fish rows inside the fish columns, so other appearances in those columns are impossible.",
      "practiceGoal": "Practice extending X-Wing thinking from two rows/columns to three rows/columns.",
      "related": [
        "x-wing",
        "naked-triple",
        "xy-wing"
      ],
      "keywords": [
        "swordfish sudoku",
        "sudoku fish pattern",
        "advanced sudoku swordfish"
      ],
      "faqs": [
        {
          "question": "What is a Swordfish in Sudoku?",
          "answer": "A Swordfish is a three-row/three-column fish pattern for one candidate number."
        },
        {
          "question": "Is Swordfish harder than X-Wing?",
          "answer": "Yes. Swordfish is the three-line version of the same fish logic used by X-Wing."
        },
        {
          "question": "Which candidate do I track in Swordfish?",
          "answer": "Track one candidate at a time. Do not mix different numbers inside the same fish pattern."
        },
        {
          "question": "Do beginners need Swordfish?",
          "answer": "Usually no. Master singles, pairs, triples, and X-Wing before relying on Swordfish."
        }
      ]
    },
    {
      "title": "XY-Chain in Sudoku: How to Spot It",
      "slug": "xy-chain",
      "url": "https://sudoku.franklab.us/techniques/xy-chain/",
      "description": "Learn the XY-Chain Sudoku technique: linked bivalue cells prove a candidate cannot stay in a cell that sees both chain ends.",
      "level": "Expert",
      "order": 12,
      "category": "chain",
      "summary": "An XY-Chain links bivalue cells so that both possible chain outcomes force the same candidate at one end, allowing an elimination.",
      "quickAnswer": "An XY-Chain in Sudoku is a chain of bivalue cells connected by shared candidates. If both ends of the chain imply the same candidate, any cell that sees both ends cannot contain that candidate.",
      "prerequisites": [
        "xy-wing",
        "naked-pair"
      ],
      "recognitionRules": [
        "Mark cells with exactly two candidates.",
        "Build a chain where neighboring cells share one candidate and alternate possibilities.",
        "Check whether both chain ends contain the same candidate.",
        "Remove that candidate from any cell that sees both ends."
      ],
      "eliminationRule": "At least one chain endpoint must become the shared candidate, so a cell seeing both endpoints cannot keep that candidate.",
      "practiceGoal": "Practice short chains first; if you cannot explain both endpoints, do not make the elimination.",
      "related": [
        "xy-wing",
        "x-wing",
        "swordfish"
      ],
      "keywords": [
        "xy-chain sudoku",
        "sudoku chains",
        "expert sudoku technique"
      ],
      "faqs": [
        {
          "question": "What is an XY-Chain in Sudoku?",
          "answer": "An XY-Chain is a linked sequence of bivalue cells used to eliminate a candidate seen by both ends of the chain."
        },
        {
          "question": "How is XY-Chain related to XY-Wing?",
          "answer": "An XY-Wing is a short three-cell version of XY-chain logic."
        },
        {
          "question": "Do all cells in an XY-Chain need two candidates?",
          "answer": "For the standard XY-Chain, yes. Each link should be a bivalue cell."
        },
        {
          "question": "Why are XY-Chains expert-level?",
          "answer": "They require tracking a conditional chain across multiple cells, which is much easier to misread than local pairs or triples."
        }
      ]
    }
  ]
}