The Square Four is a motorcycle produced by Ariel between 1931 and 1959, designed by Edward Turner, who devised the Square Four engine in 1928. At this time he was looking for work, showing drawings of his engine design to motorcycle manufacturers. The early engine with « two transverse crankshafts » was essentially a pair of ‘across frame’ OHC parallel twins joined by their geared central flywheels, with a four-cylinder block and single head. The idea for the engine was rejected by BSA, but adopted by Ariel. Thus it became the Ariel Square Four.