Notes in Sudoku

The moment you admit the puzzle is difficult.

Notes — also called candidates or pencil marks — are small digits written inside an empty cell to record which values could still legally go there, given everything already placed on the board. For every empty cell, a digit is a valid candidate unless it appears somewhere in that cell's row, column, or 3×3 box.

As you fill in more of the grid, some candidates stop being valid and must be removed from every cell they touch — Evil Sudoku strips them automatically the instant you place a digit, so your notes stay accurate without manual bookkeeping. Nearly every intermediate and advanced technique on this site — obvious singles, hidden pairs, pointing pairs, X-Wing, and beyond — works by reading patterns out of these candidate notes rather than the board's placed digits. Without accurate notes, none of those patterns are visible, and a solver is reduced to guessing on anything beyond the easiest puzzles.

How It Works

For every empty cell, write down all digits that are not yet forbidden by its row, column, or box. These are its candidates.

As you place digits elsewhere on the board, some candidates become impossible. Evil Sudoku automatically strips them from every affected cell's notes for you the moment the digit is placed. Eventually, cells with only one remaining candidate reveal themselves as obvious singles and can be filled in immediately.

Notes are the foundation on which every intermediate and advanced technique is built. Without accurate notes, you are guessing.

Example

Below board presents an example of notes recorded for the empty cells of a box, alongside the digits 2, 6, and 9 already placed there.

Three cells in this box already hold digits: 2, 6, and 9. Every empty cell's notes skip those three — they are no longer candidates anywhere in the box. What remains in each cell's notes are its true possibilities, given everything known so far.

On Evil Boards

Evil boards are beyond human capacity to solve without using notes since they enable intermediate and advanced pattern matching techniques.