Border 1-2-1
A 1-2-1 run on the border forces the middle border edge and rules out both 1s' inward and outer sides.
How it works
- 1The classic "1-2-1", checked as one run of three rather than pair by pair — some ways of satisfying any two of these clues still fail the third.
- 2The run sits against the top of the board, which is what makes it bite: those border dots have only three edges each, and all of them belong to these three cells, so the "0 or 2 lines" rule can be applied inside the window.
- 3Work through the combinations that survive and the middle 2 has its top drawn in every one, while each 1 is left holding its single line on either its top or the side it shares with the 2.
- 4Draw the 2's top, and cross off both 1s' bottoms and both their outer sides. Try this same run in the middle of a board and it forces nothing at all — the border is the whole trick.
- Edge the technique adds
- Edge ruled out
- Cells that establish the pattern