Edge Forcing

Level: Basic

If a cell at the end of a line is already filled, the block on that side is fixed.

If the very end cell of a line is already filled, then that line's first (or last) clue block must start from that end. So you can fill in cells equal to the block's length in a row, and block off the next cell with an X.

For example, if the left end cell is filled and the first clue is 3, the left three cells are all filled and the fourth cell must be empty (X).

Edges have little freedom, so they are places where confirmations come easily. If a crossing line has already confirmed an end cell, it leads straight into this technique.

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