How to Read Sudoku Notation

Rows, columns and cells

Rows are numbered 1 to 9 from top to bottom. Columns are numbered 1 to 9 from left to right. A single cell is named by its row and column together. The common shorthand is r for row and c for column, so r4c7 means the cell in the fourth row and the seventh column. Some writers flip it and write the column first, but the meaning is the same.

Boxes

The nine three by three blocks are called boxes. They are usually numbered 1 to 9 in reading order, so box 1 is the top left, box 3 is the top right, box 5 is the centre, and box 9 is the bottom right. When a guide says a digit is locked in box 5, it means confined to that centre block.

Units, peers and candidates

• A unit is any row, column or box, the three groups where each symbol appears once.
• Two cells are peers if they share a unit, since one of them being a value rules it out for the other.
• A candidate is a value that could still go in a cell, the small pencil marks you keep as notes.
• A conjugate pair is a unit where a digit has just two candidate cells left.

Putting it together

With that vocabulary a sentence like remove 5 from r2c3 and r2c8 because of an X-Wing on columns 3 and 8 becomes perfectly clear. Notation is not showing off. It is a shared shorthand that lets a writer describe a precise move in a few characters, and lets you follow along on your own board.