Mirrored from Sudopedia, the Free Sudoku Reference Guide
When a solving technique only deals with the candidates for a single digit, it is customary to use the letter X to replace this digit, both in chain notation and in the illustrating diagrams. This abstraction helps the reader understand that this is a single-digit technique and focus on the pattern rather than the pencilmarked candidates.
You are advised to use fish diagrams to illustrate these techniques. These diagrams are based on a text format used in fish-related topics in the Sudoku Players' forum.
Here is an example, illustrating a Finned Swordfish:
This is the same example in the text format used in the forums:
. / . | . / . | . / . * X * | . X . | . X . . F . | . / . | . / . --------+---------+-------- . / . | . / . | . / . . X . | . X . | . X . . / . | . / . | . / . --------+---------+-------- . / . | . / . | . / . . X . | . X . | . X . . / . | . / . | . / .
If you do not have the tools to create graphical fish diagrams, use the text version instead. Sooner or later, a helpful Sudopedian will replace it with a graphical version.
There are only a few symbolic characters used in fish diagrams.