Three squares in a row

Your eyes should light up when you see a row or column of three in a region, because it is very useful for solving squares.

You can immediately use the block to exclude possibilities.

In the artificial grid alongside you can see that I've put 1,2,3 in the central region.

In the region to the left I've filled in two rows just to illustrate.

Because there is this central row the allocations in the right hand region are now fairly fixed even though no squares are allocated. We know that 7;8;9 must occupy the central row to complete row E. However more significantly we can tell that 1;2 and 3 must occupy the top row of this region (in any order though) and 4;5;6 occupy the bottom row. That means by the magic of the block of 3, each square in a blank region is knocked down to only 3 rather than 9 possibilities.

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The key reason why a block of three in a row or column is that it forces allocation in just one adjacent row or column set of three, narrowing down the options substantially.

Also if you look along the block of three into the adjacent Sudoku regions you know that all these numbers must occur in either of the remaining two sub-rows/columns.

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