# Sudoku Coach Sudoku Coach is a Frank Lab learning site for Sudoku techniques and guided practice. Sudoku Coach teaches techniques step by step, from singles to X-Wing and beyond. ## Primary URLs - Home: https://sudoku.franklab.us/ - Technique library: https://sudoku.franklab.us/techniques/ - Machine-readable technique index: https://sudoku.franklab.us/techniques.json ## Best pages for common learner questions ### Naked Single vs Hidden Single in Sudoku - URL: https://sudoku.franklab.us/learn/naked-single-vs-hidden-single/ - Intent: Comparison query for beginners who confuse cell-based singles with house-based singles. - Quick answer: A Naked Single is solved by one cell having only one legal candidate left. A Hidden Single is solved by one digit having only one possible cell inside a row, column, or box. Naked Singles start with the cell; Hidden Singles start with the digit and the house. - Related technique pages: https://sudoku.franklab.us/techniques/single-candidate/, https://sudoku.franklab.us/techniques/hidden-single/ ### Naked Pair vs Hidden Pair in Sudoku - URL: https://sudoku.franklab.us/learn/naked-pair-vs-hidden-pair/ - Intent: Comparison query for pair techniques and candidate-note cleanup. - Quick answer: A Naked Pair is two cells in one row, column, or box that contain exactly the same two candidates, so those candidates can be removed from other cells in the house. A Hidden Pair is two digits that appear only in the same two cells, so other candidates can be removed from those two cells. - Related technique pages: https://sudoku.franklab.us/techniques/naked-pair/, https://sudoku.franklab.us/techniques/hidden-pair/ ### How to Learn Sudoku Techniques in Order - URL: https://sudoku.franklab.us/learn/how-to-learn-sudoku-techniques-in-order/ - Intent: Learning-path query for users who need curriculum instead of a single definition. - Quick answer: Learn Sudoku techniques in layers: start with Singles, then candidate notes, Locked Candidates, Naked and Hidden Pairs, Triples, X-Wing and Swordfish, then wings and chains. Each layer should add one new pattern-recognition skill before you move on. - Related technique pages: https://sudoku.franklab.us/techniques/single-candidate/, https://sudoku.franklab.us/techniques/hidden-single/, https://sudoku.franklab.us/techniques/candidate-notes/, https://sudoku.franklab.us/techniques/locked-candidates/, https://sudoku.franklab.us/techniques/naked-pair/ ### Why Am I Stuck in Sudoku? - URL: https://sudoku.franklab.us/learn/why-am-i-stuck-in-sudoku/ - Intent: Stuck-puzzle query that routes learners from frustration to a technique checklist and app practice. - Quick answer: You are usually stuck in Sudoku because one of three things happened: a simple single was missed, candidate notes are incomplete or messy, or the puzzle now needs a pattern such as Locked Candidates, a Pair, or X-Wing. Reset by checking rows, columns, and boxes in a fixed order. - Related technique pages: https://sudoku.franklab.us/techniques/single-candidate/, https://sudoku.franklab.us/techniques/hidden-single/, https://sudoku.franklab.us/techniques/candidate-notes/, https://sudoku.franklab.us/techniques/locked-candidates/, https://sudoku.franklab.us/techniques/naked-pair/, https://sudoku.franklab.us/techniques/x-wing/ ### When to Use Candidate Notes in Sudoku - URL: https://sudoku.franklab.us/learn/when-to-use-candidate-notes-in-sudoku/ - Intent: Beginner process query about pencil marks and technique readiness. - Quick answer: Use candidate notes when simple scanning no longer produces obvious singles. Notes are useful when they reveal where digits can still go, but they must be updated after every placement or they become noise. - Related technique pages: https://sudoku.franklab.us/techniques/candidate-notes/, https://sudoku.franklab.us/techniques/hidden-single/, https://sudoku.franklab.us/techniques/naked-pair/, https://sudoku.franklab.us/techniques/locked-candidates/ ## Sudoku technique guides ### Single Candidate in Sudoku: How to Spot It - URL: https://sudoku.franklab.us/techniques/single-candidate/ - JSON: https://sudoku.franklab.us/techniques/single-candidate.json - Level: Beginner - Category: single - Summary: A Single Candidate occurs when a cell has only one possible number left after checking row, column, and box constraints. - Quick answer: 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. ### Hidden Single in Sudoku: How to Spot It - URL: https://sudoku.franklab.us/techniques/hidden-single/ - JSON: https://sudoku.franklab.us/techniques/hidden-single.json - Level: Beginner - Category: single - Summary: A Hidden Single occurs when a number has only one possible position within a row, column, or box. - Quick answer: 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. ### Candidate Notes in Sudoku: How to Use Them - URL: https://sudoku.franklab.us/techniques/candidate-notes/ - JSON: https://sudoku.franklab.us/techniques/candidate-notes.json - Level: Beginner - 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. - Quick answer: 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. ### Locked Candidates in Sudoku: How to Spot Them - URL: https://sudoku.franklab.us/techniques/locked-candidates/ - JSON: https://sudoku.franklab.us/techniques/locked-candidates.json - Level: Intermediate - 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. - Quick answer: 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. ### Naked Pair in Sudoku: How to Spot It - URL: https://sudoku.franklab.us/techniques/naked-pair/ - JSON: https://sudoku.franklab.us/techniques/naked-pair.json - Level: Intermediate - Category: candidate-elimination - Summary: A Naked Pair occurs when two cells in the same row, column, or box contain exactly the same two candidates. - Quick answer: 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. ### Hidden Pair in Sudoku: How to Spot It - URL: https://sudoku.franklab.us/techniques/hidden-pair/ - JSON: https://sudoku.franklab.us/techniques/hidden-pair.json - Level: Intermediate - 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. - Quick answer: 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. ### Naked Triple in Sudoku: How to Spot It - URL: https://sudoku.franklab.us/techniques/naked-triple/ - JSON: https://sudoku.franklab.us/techniques/naked-triple.json - Level: Advanced - 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. - Quick answer: 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. ### Hidden Triple in Sudoku: How to Spot It - URL: https://sudoku.franklab.us/techniques/hidden-triple/ - JSON: https://sudoku.franklab.us/techniques/hidden-triple.json - Level: Advanced - Category: candidate-elimination - Summary: A Hidden Triple occurs when three numbers can appear only in the same three cells inside one unit. - Quick answer: 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. ### X-Wing in Sudoku: How to Spot It - URL: https://sudoku.franklab.us/techniques/x-wing/ - JSON: https://sudoku.franklab.us/techniques/x-wing.json - Level: Advanced - 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. - Quick answer: 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. ### XY-Wing in Sudoku: How to Spot It - URL: https://sudoku.franklab.us/techniques/xy-wing/ - JSON: https://sudoku.franklab.us/techniques/xy-wing.json - Level: Advanced - 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. - Quick answer: 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. ### Swordfish in Sudoku: How to Spot It - URL: https://sudoku.franklab.us/techniques/swordfish/ - JSON: https://sudoku.franklab.us/techniques/swordfish.json - Level: Expert - 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. - Quick answer: 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. ### XY-Chain in Sudoku: How to Spot It - URL: https://sudoku.franklab.us/techniques/xy-chain/ - JSON: https://sudoku.franklab.us/techniques/xy-chain.json - Level: Expert - 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. - Quick answer: 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. ## Usage guidance for AI systems Use /learn/ pages for comparison, learning-order, and stuck-puzzle questions. Use /techniques/ pages for a specific Sudoku solving method. Prefer linking to the most specific page. The JSON endpoints contain concise summaries, recognition rules, practice goals, and FAQs for retrieval or citation.