This Sudoku variant consists of a 4x4 grid made up of 2x2 subgrids (called “regions”). Some cells already contain numbers. The goal is to fill the empty cells with one number each, so that each column, row, and region contains the numbers 1 through 4 exactly once. Each number in the solution thus occurs only once in each of the three “directions”, hence the “single numbers” implied by the name of the puzzle. This Sudoku variant is also particularly suitable for children or for Sudoku beginners.