Pencilmark

When solving a Sudoku, it is impossible to remember which candidates are available for each cell. Yet, many solving techniques require that you know which candidates are available.

Placing pencilmarks is one solution to this problem. The remaining candidates are pencilled in small digits in the edges of the cell, or placed as small dots in a 3x3 pattern, each position representing a digit.

Most helper programs offer a tool that maintains pencilmarks automatically.

A pencilmark grid or PM grid is a diagram in which all the remaining candidates are shown. Annotations are often used to show which cells are involved in the next solving step.

This is an example of a pencilmark grid:

.------------------.------------------.------------------.
| 19    7     169  | 2     156   3    | 459   8     49   |
| 3     4     5    | 7     9     8    | 1     2     6    |
| 2     169   8    | 16    4     156  | 3579  35    379  |
:------------------+------------------+------------------:
| 4     136   1367 | 5     1367  9    | 8     36    2    |
| 17    5     12367| 8     12367 4    | 367   9     37   |
| 8     369   23679| 36    2367  26   | 34567 3456  1    |
:------------------+------------------+------------------:
| 1579  139   13479| 13469 12356 1256 | 23469 346   8    |
| 159   8     1349 | 13469 12356 1256 | 23469 7     349  |
| 6     2     349  | 349   8     7    | 349   1     5    |
'------------------'------------------'------------------'

Complementary Pencilmarks

Some players use pencilmarks not to represent remaining candidates, but the eliminated candidates. This reduces the need for an eraser, but it requires you to look at the grid from a different perspective.