Singles

A single sets a digit immediately. After each candidate removal, look for singles before moving on to more advanced techniques.

Full House

A row, column or box has eight filled cells and only one empty cell. The empty cell must contain the missing digit.

In the example above box 1 has eight filled cells. The remaining cell R2C1 must be 1 (shown in green).

One Digit Left

The grid already contains eight copies of a digit. The last copy must go in the only cell that can still take it.

In the example above eight 1s are already placed. The only remaining cell for 1 is R1C9 (shown in green).

Naked Single

A cell has only one candidate left. That candidate must be the digit for the cell.

In the example above R2C1 has only 5 left as a candidate.

Hidden Single

A digit has only one possible cell in a row, column or box. Even if that cell has other candidates, the digit must go there.

In the example above 9 can only go in R3C2 in row 3. The other cells in row 3 are blocked by the remaining 9s in the grid.