A straight-twin engine, also known as an inline-twin, vertical-twin, or parallel-twin, is a two-cylinder piston engine whose cylinders are arranged in a line along a common crankshaft.

Engine balance refers to how the inertial forces produced by moving parts in an internal combustion engine or steam engine are neutralised with counterweights and balance shafts, to prevent unpleasant and potentially damaging vibration. The strongest inertial forces occur at crankshaft speed and balance is mandatory, while forces at twice crankshaft speed can become significant in some cases.