GridWits
Medium tier Technique 12 of 31 Used by Smart Hint

Border 1-3

A 1 and 3 sharing a border dot force edges that the same pair in the board's interior would leave open.

How it works

  1. 1Take the two clues together and try every way of drawing the seven edges around them, keeping only the ways that give the 1 exactly one line, the 3 exactly three, and leave every dot with a legal count.
  2. 2The dot between them is the lever: it sits on the board's top border, so it has only three edges and all three belong to these two cells — the window can apply the "0 or 2 lines" rule right there.
  3. 3Split on the shared edge. If it is a line the 1 is already finished, so its top is blank and that dot has to take the 3's top; if it is blank the 3 needs its other three sides, and then the dot has to take the 1's top instead.
  4. 4Both branches agree: the 3's top is a line, and the 1's bottom and left are out. Every named shape below this entry is this same check on a particular pair of clue values.
1 3
  • Edge the technique adds
  • Edge ruled out
  • Cells that establish the pattern
Next in the ladder Border Clue and Dot →

One clue combines with a nearby border dot, whose missing outside edge reduces its possibilities.