Tandem cylinders are two cylinders on the same throw. The inboard cylinder may be the high pressure or low pressure cylinder, and is single acting on the crank end only. The outboard cylinder may also be the high pressure or low pressure cylinder, and is single acting on the head end only.

Tandem cylinders are used to allow two stages of compression on a single throw. Since they are two single acting cylinders, flow rates are smaller than double acting cylinders.