Hidden Singles Sudoku Strategy

How the strategy works

Choose a number that already appears several times on the board. Check one row, column, or 3x3 box and ask whether that number has only one legal place left.

The square may not look obvious at first because other numbers could also seem possible there. The hidden single is about the number's only place in that area.

Large print practice routine

Use this technique slowly on a readable board or a printed sheet. The goal is to remove uncertainty, not to solve faster.

After each candidate cleanup, return to scanning and singles before looking for another advanced pattern.

Hidden singles are usually worth checking before candidate-heavy techniques because they often appear after one new number is placed. On a large print board, use the visible rows and boxes as anchors so the scan stays calm and repeatable.

• Pick one missing number in a nearly full box.
• Cross out squares blocked by the same number in a row or column.
• If one square remains, place the number.
• Repeat slowly instead of jumping to a harder technique.

Example pattern

Example: a 3x3 box is missing 1, 4, and 8. The number 4 already appears in two crossing rows, so two empty squares in that box cannot hold 4. If the third square is the only legal place left, 4 is a hidden single for that box.

The square may still look busy because it could hold other numbers from a square-by-square view. The key is that 4 has no other home inside the area you are checking.

Common mistakes to avoid

Do not call a move hidden just because the square looks likely. The proof comes from checking every possible place for one number inside one row, column, or box.

Do not scan the whole board at once. Hidden singles are easier to see when the player chooses one number and one area, then finishes that check before moving on.