Hidden Single

This is when a square can contain only one possible number because the number cannot exist in any other square in that row, column or block. Consider the following board:

The highlighted square can only contain a 7.  This is because the 7's in the first and third blocks prevent another 7 from being placed in the first and third rows. This means for the second block, a 7 can exist only in the second row. There are only two free squares in the second row. However, the 7 in block five prevents another 7 from being placed in column four. Therefore there is only one free square in block two, which is where a 7 must go. This is what people do when they use the term 'cross-hatching'.

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