This is performed from the outset and throughout the solution. Some of the easier puzzles can be solved by this method alone. It may be necessary to scan several times and consists of two basic techniques:

Cross-hatching:

This is performed by the scanning of rows or columns to identify which line in a region may contain a certain number. This is done by process of elimination and for the fastest results the numbers are scanned in order of their frequency. It is important that this is performed systematically with all digits 1-9 being checked.

Counting 1-9 in regions, rows, columns to identify missing numbers:

Counting based on the last number discovered may speed up the process. With the more difficult puzzles especially it can often be the case that the easiest way to discover the number for a cell is by counting in reverse. That is by scanning the cell?s region, row and column to find the numbers it cannot be, to see which is left.