Generally speaking, 1/4,3/8 and 1/2 refer to the square joint edge of the wrench,1/4 refers to the square head size of 6.3MM, 3/8 refers to 9.5MM, and 1/2 refers to12.5mm.

1/4 fly wrench is especially suitable for 14MM socket nut, 3/8 fly wrench is suitable for working with sockets within 24MM, and 1/2 fly wrench is more suitable for working with sockets within 32 mm