How it works
Pencil marks turn a sudoku from a single-decision puzzle into a constraint-elimination puzzle. Each empty cell lists every digit that could go there, and you remove candidates as the grid fills.
There are two notation styles in common use. Positional style places each digit in the cell at the position it would occupy in a mini 3x3 grid. List style just writes the candidates in a row. Positional reads faster once you are used to it.
Marks must stay accurate. Every time you place a digit, remove that digit from the candidate sets of cells it can see. Drifting pencil marks are the biggest cause of getting stuck.
When to look for it
When a full scan of the grid stops finding singles. Pencil marks unlock pair, triple and chain techniques.
Tips for spotting the pattern
- Mark every empty cell, or mark none. Half-marked grids trick you into trusting bad information.
- Sweep peers immediately after every placement. The slow part is up front; updates are cheap.
- Switch to digital pencil marks if your eraser is wearing out the paper.
Common mistakes
- Pencil-marking too early. Easy puzzles do not need them and they slow you down.
- Leaving stale marks after placements. The candidate you trust may already be eliminated.
- Mixing notation styles mid-puzzle. Stick to one approach per solve.