The Ariel Square Four motorcycle that was released in 1958 was a bike with a unique design. It had two parallel-mounted twin cylinders 500 cc engines that were mounted on a common block. Since all the four cylinders were arranged in a square pattern, this 1000 cc bike was labelled as Square Four.

Designed by a motorcycle agent, Edward Turner, he first tried to sell his idea to BSA but had no encouraging results, therefore prompting him to go to Ariel whose director decided to employ Turner.

A little over 2500 were produced.