I keep coming back to Quadruples, powerful little circles providing information about four cages at once. Sometimes the information might only be about two digits, but it’s still information about the four digits touching the circle. Powerful clue, quads, and easy to understand, though they can be tricky to master.
Part of Quad University
Rules:
Normal Sudoku rules apply.
Estimated difficulty: 1 out of 5
Each digit within circles appears at least once in cells touching the circle for each time it appears within the circle.
