Basic Techniques: Full House, Cross-Hatching & Hidden Singles
Learn the most fundamental Sudoku techniques with diagrams and an interactive practice board: Full House, Cross-Hatching, and Hidden Single.
1. Full House (Last Empty Cell in a Unit)
Full House is the simplest deduction in Sudoku. It occurs when any row, column, or 3×3 block has 8 digits already filled and only 1 empty cell remaining. Since every unit must contain digits 1 through 9 exactly once, the missing 9th digit is 100% determined.
Interactive Practice: Full House
Look at the top row. Digits 1 through 8 are placed. Tap the empty top-right cell and enter the missing number!
A Hidden Single occurs when a specific digit can only go into one single cell within a 3×3 block, row, or column because all other empty cells in that unit are blocked by intersecting lines containing that same digit.
