Pinned Squares

A pinned square is one that is the only place a particular number can go in a row, column or 3x3 block and is basically the full version of intersections.

For example, by using 'Show squares with a...' and selecting <1>, it is clear that R9C5 must be a <1>. It is the only place the <1> can go in Block8 and so <1> is pinned to R9C5.

Using 'Show squares with a...' and selecting <5>, <5> is pinned to R2C8 as this is the only <5> in R2. Similarly. <5> is also pinned to R3C6 as it is the only <5> in Block2.

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