Extra Regions Sudoku Explained in One Short Example
A shaded diagonal of three cells works like an extra box. Two 5s in the region break the rule, and 5, 1 and 8 pass because they are all different.
Transcript
The rule
Extra regions sudoku shades groups of cells in the grid. Each shaded region follows the box rule, so no digit can repeat inside it. A nine-cell region holds 1 to 9 exactly once.
Extra regions give you another group to scan for singles. Because they cut across rows, columns and boxes, they link parts of the grid that the normal rules keep apart.
Worth memorizing
A 9-cell extra region holds 1 to 9 exactly once, just like a box.
Regions that cross boxes create new places for singles to appear.
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