Diagonal Sudoku Puzzle Books Diagonal Sudoku Puzzle Books - 400 Easy to Master Puzzles 6x6 (Volume 1), Book 1, (Paperback)

Diagonal Sudoku also called Sudoku X.

The puzzle is a 6x6 grid made up of 3x2 subgrids (called "regions").
Some cells already contain numbers, known as "givens".

The goal is to fill in the empty cells, one number in each, so that each column, row, region and diagonal contains the numbers 1 through 6 exactly once.

Each number in the solution therefore occurs only once in each of three "directions", hence the "single numbers" implied by the puzzle's name.