The pistons are usually mounted on the crankshaft so that opposing pistons move back and forth in opposite directions at the same time, somewhat like boxing competitors punching their gloves together before a fight, which has led to it being referred to as a "boxer" engine. The design is rarely seen with shared crank throws (See Coventry Climax FWMW for such a non-boxer flat engine), so "flat-four" and "boxer-four" are usually used synonymously.