A full house is the friendliest move in Sudoku: a row, column, or box with exactly one empty cell. Eight of the nine digits are already in place, so the remaining cell has no choice — it takes the one digit the house is still missing.
To spot one, scan for houses that are nearly complete. A row with a single gap, a box with one blank square — count what is there, and whatever digit is absent belongs in the gap.
It works because of the one rule everything else in Sudoku grows from: every row, every column, and every box contains each digit 1 through 9 exactly once. With eight placed, the ninth is forced.
On the board
On this board, row 1 is one cell short of complete: reading across, it already holds 9, 3, 8, 1, 7, 6, 4, and 2, and only the cell at row 1, column 7 is empty. The single digit missing from the row is 5 — so R1C7 must be 5. The washed row is the house being counted, and the glowing digit is the placement the technique forces.
This is the exact board the app's interactive Full House demo uses — step 1 of its 21-step walkthrough, drawn here with its live pencil marks.
