Sudoku · techniques

Five patterns. In this order.

Almost every Sudoku falls to a chain of these.

Naked single

One cell has only one possible digit. Place it.

Row 1 contains every digit except 4. So C1 must be 4.

Hidden single

Within a row, column, or box, only one cell can hold a given digit. Place it there — even if that cell has other candidates too.

Within the top-right box, only H1 can hold a 7 — every other cell in the box has 7 blocked by a peer.

Naked pair

Two cells in a unit share the same two candidates and only those. Eliminate that pair from every other cell in the unit.

A4 and C4 must hold 3 and 7 between them — so 3 and 7 can be erased from every other cell in row 4.

Hidden pair

Two digits only fit in two cells of a unit. Those cells must be that pair — clear all other candidates from them.

2 and 5 can only go in B5 and F5 — so those cells must be 2 and 5 (in some order). All other candidates in those cells can go.

X-wing

A digit appears in exactly two columns of two different rows — and those columns match. The digit must go in those columns in those rows. Clear it from those columns elsewhere.

6 can only go in columns B and G across rows 2 and 7. So 6 must be in those columns in those rows — clearing 6 elsewhere in B and G.

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