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Naked Single
EasyA naked single is an empty cell that has only one candidate digit left, once you cross off every digit already placed in its row, column, and 3×3 box.
It's the most basic solving step and the one every other technique eventually reduces to: once a cell has a single candidate, it must be that digit.
Say a cell's row already contains 1, 2, 4, 6, and 8, its column contains 3, 5, and 7, and its box adds nothing new — every digit from 1 through 8 is accounted for, so the only digit left standing is 9. Most solvers sweep the whole board for naked singles first, since placing one can immediately create another right next to it.
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Every other cell in this box already has a digit — only the highlighted cell is empty.
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