Boards with the Quadruple variant have circles at some junctions of the puzzle. Each circle contains 1 to 4 digits, and these digits must appear in the four surrounding cells. Several strategies can assist when solving puzzles that include the Quadruple variant.

Tip: When a quadruple includes the same digit twice, these digits must be placed diagonally from each other.

Quadruple Strategies

1. HiddenSingle

Level: Simple

If a digit can only fit in one cell within the quadruple, it must go there. Additionally, if the quadruple has two identical digits and one of them must be placed in a cell around the quadruple, the second digit should be placed in the diagonal cell.

Example:

Look at the highlighted quadruple in the image below. The digit 5 must be in one of the cells around the quadruple. It cannot go in A1 and A2 due to existing constraints, and it cannot go in B2 (because of the arrow). Therefore, B1 must be 5.

2. LockedSet

Level: Medium

If a cell outside the four cells of a quadruple sees all the cells of the quadruple, you can eliminate those digits from that cell.

Example:

Look at the highlighted quadruple in the image below. The digit 9 must be in one of the cells around the quadruple. Cells G9 and I7 see all the cells of the quadruple.

Therefore, 9 can be eliminated from G9 and I7.

3. DigitLockedSet

Level: Advanced

If a cell outside the quadruple can see all the cells where a quadruple digit could go, you can eliminate this digit from that cell.

Example:

Look at the highlighted quadruple in the image below. The digit 3 must be in the cells around the quadruple and must be placed in B4 and C4. Cells A4, A5, B5, D4, and F4 have 3 as a candidate and see the cells of the quadruple where 3 must go.

Therefore, 3 can be eliminated from cells A4, A5, B5, D4, and F4.