81Sudoku
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81Sudoku
Every 81Sudoku puzzle is rated by the exact technique it takes to crack it. Here's what each of those techniques actually means, how to spot it, and a worked example.
When a digit inside one box only fits in cells that share a row or column, it can be crossed off the rest of that row or column outside the box - the box itself is 'pointing' the digit into a line.
Read the explainer →The mirror of pointing pairs: when a digit inside one row or column only fits inside a single box, it can be crossed off the rest of that box - the line 'claims' the digit for that box.
Read the explainer →Two cells in the same row, column, or box that each have exactly the same two candidates left. Between them they must use up both digits, so no other cell in that house can hold either one.
Read the explainer →Two digits that, within a house, only have room to go in the same two cells - even if those cells still list other candidates too. Once spotted, every other candidate can be stripped from just those cells.
Read the explainer →Three cells in one house whose candidates, combined, add up to only three digits total - even if no single cell lists all three. Those digits can be crossed off everywhere else in the house.
Read the explainer →Three digits that, within a house, only have room to go in the same three cells - even if those cells still list other candidates. Every other candidate can then be stripped from just those three cells.
Read the explainer →A digit confined to the same two columns across two rows (or the same two rows across two columns), forming a rectangle. The digit must occupy those columns in both rows, so it can be cleared elsewhere in them.
Read the explainer →Three two-candidate cells: a pivot with candidates {X,Y}, and two pincers sharing a house with the pivot, holding {X,Z} and {Y,Z}. Whichever value the pivot ends up being, one pincer is forced to Z.
Read the explainer →Like an XY-Wing, but the pivot cell has three candidates {X,Y,Z} instead of two, and the pivot itself is also part of the elimination.
Read the explainer →Two cells, anywhere on the board, that share the exact same two candidates {X,Y}, linked by a chain where X can only be one of two places. That link forces one of the two cells to be Y.
Read the explainer →X-Wing's three-line version: a digit confined, across three rows, to the same three columns overall. It can be cleared from the rest of those three columns.
Read the explainer →Swordfish stretched to four lines: a digit confined, across four rows, to the same four columns overall - rare, but the same logic as X-Wing and Swordfish, just bigger.
Read the explainer →When a cell's few candidates each get followed forward through their own chain of forced consequences, and every branch agrees on some outcome elsewhere, that outcome must be true regardless of which candidate is actually correct.
Read the explainer →