The knight’s tour, as a puzzle

One rule: the knight moves two squares in any direction, then turns at a right angle and moves one more. The goal is to visit every square of the board exactly once, and never to step on a square already used.

Five boards, from 6×6 to 10×10, and three levels of help — from a mode that lights up every legal move to a bare board where you count for yourself. It plays in the browser, with no sign-up.

The problem is close to a thousand years old. The earliest written tours are from the ninth century, Euler took it up in 1759, and the rule that finds a tour almost every time is Warnsdorff’s, from 1823.