Jigsaw

A Jigsaw Sudoku does not have 3x3 boxes, but regions with an irregular shape.

A Jigsaw example:

.-------.---------------.-----------.
| .   8 | .   .   .   . | .   .   1 |
|       '-------.       |           |
| 1   .   .   . | .   6 | 3   .   . |
|       .---.---:       '---.       |
| .   . | . | . | .   .   . | 4   . |
|   .---'   |   '---.-------'---.   |
| . | .   . | 3   . | 8   .   7 | . |
:---'       |       '---.       '---:
| .   .   . | .   8   . | .   .   . |
:---.       '---.       |       .---:
| . | 7   .   1 | .   2 | .   . | . |
|   '---.-------'---.   |   .---'   |
| .   5 | .   .   . | . | . | .   . |
|       '---.       :---'---'       |
| .   .   7 | 9   . | .   .   .   4 |
|           |       '-------.       |
| 3   .   . | .   .   .   . | 1   . |
'-----------'---------------'-------'

The irregular regions allow the player to use the Law of Leftovers, a solving technique which can only be used on irregular regions.

Jigsaw is one of the most popular Sudoku Variations