Technique guide
Hidden single
A hidden single is the only cell in a unit that can hold a particular digit. The cell may show several candidates, but the digit has no other possible home in that row, column, or box.
When to use it
Look for it when no cell has a single candidate. Change the question from “what fits here?” to “where can this digit go in this unit?”
A small example
In one 3×3 box, digit 4 is missing. Two empty cells are blocked by a 4 in their columns and another is blocked by a 4 in its row. The one remaining position must be 4, even if its notes also include 2 and 7.
Step by step
- 1Select one row, column, or box and identify a missing digit.
- 2Cross out empty cells blocked by that digit in intersecting units.
- 3If one position survives, place the digit there and verify all three units.
Recommended level: Beginner to intermediate.
Put it into practice
Practice scanning by digit on an Easy board.