A Sudoku Glossary: Every Common Term Explained

The board itself

• Cell: one of the 81 small squares in the grid, each of which holds a single value once the puzzle is solved.
• Row: a horizontal line of nine cells running across the grid, and one of the three kinds of group that must contain every value once.
• Column: a vertical line of nine cells running down the grid, subject to the same one of each rule as a row.
• Box or region: one of the nine 3 by 3 blocks, marked by heavier borders, that must also hold every value exactly once.
• Group or unit: any row, column or box, a handy catch all word for the three places where the no repeats rule applies.
• Peer: any other cell that shares a row, column or box with a chosen cell, and so helps decide that cell's candidates.
• Grid: the whole 9 by 9 playing field, made up of nine rows, nine columns and nine boxes.
• Clue count: the number of shapes shown at the start, a rough and often misleading guide to how hard a board will feel.

Starting position and goal

• Clue or given: a value filled in before you start that cannot be changed, shown in Shapedoku as a locked shape that anchors the board.
• Solution: the one correct, fully completed grid that a good puzzle is guaranteed to have.
• Unique solution: the promise that only a single completed grid fits the clues, which is what lets a puzzle be solved by pure logic with no guessing.
• Symmetry: a balanced, pleasing arrangement of the starting clues that is purely cosmetic and has no effect on the solving logic.
• Locked shape: the Shapedoku name for a given, a starting symbol you can never change or clear.
• Editable cell: any cell that is not a given, which you are free to fill, change or clear as you solve.

Notes, candidates and cleanup

• Candidate: a value that could still legally go in a given cell, once its current row, column and box are taken into account.
• Note or pencil mark: a small candidate you jot inside an empty cell to track the possibilities, and the heart of Notes mode.
• Elimination: removing a candidate from a cell because some deduction has proven it cannot possibly belong there.
• Smart cleanup: the automatic removal of candidates that a new placement makes impossible, keeping your notes honest without any manual erasing.
• Auto notes: an optional feature that fills in every candidate for you, a shortcut some solvers prefer to switch off.
• Candidate count: how many possible values a cell still has, a quick gauge of how close it is to resolving to one.

Core solving techniques

• Scanning: sweeping across rows, columns and boxes to find a value that is forced into a single cell.
• Crosshatching: scanning one value through a single box, using the rows and columns it already occupies to pin down its last free cell.
• Naked single: a cell that has only one candidate left, so that value must go there.
• Hidden single: a value that can legally go in only one cell of a group, even though that cell may still show other candidates.
• Naked pair: two cells in the same group sharing the identical two candidates, which lets you strike those two values from every other cell in the group.
• Hidden pair: two values that can only be placed in the same two cells of a group, which lets you clear all other candidates from that pair of cells.
• Naked triple: three cells in a group sharing three candidates between them, which clears those values from the rest of the group.
• Pointing set: candidates in a box lined up on a single row or column, letting you eliminate that value along the line beyond the box.

Patterns and play

• Pointing pair or locked candidate: a value confined to one row or column within a box, which lets you remove it from the rest of that line outside the box.
• X-Wing: a rectangle of one value across two rows and two columns that lets you erase the value from the rest of those columns or rows.
• Chain: a linked run of if this then that deductions used to crack the hardest boards when simpler patterns run out.
• Mistake: a placement that breaks a rule or contradicts the solution, which costs a life in the three mistake classic mode.
• Hint: a reveal of one correct cell, with the first hint free on every Shapedoku puzzle.
• Daily Challenge and streak: one fresh puzzle shared by all players each day, and the running count of days you have kept it up.
• Swordfish: a larger cousin of the X-Wing spread across three rows and three columns of a single value.
• Coloring: tracing a value through linked cells with two colors to expose a contradiction and force a placement.
• Score: a number reflecting your time, mistakes, difficulty and hints, used to rank runs on the leaderboard.