An I4 engine, also known as an inline-four engine, is a four-cylinder piston engine where the cylinders are arranged in a line along a common crankshaft. The majority of automotive four-cylinder engines use a straight-four layout, and the layout is also very common in motorcycles and other machinery. Therefore, the term "four-cylinder engine" is usually synonymous with straight-four engines. When a straight-four engine is installed at an inclined angle (instead of with the cylinders oriented vertically), it is sometimes called a slant-four. The I4 engine is a compact engine that can fit in many engine bays, is easy to work on, and has balanced primary forces. However, the secondary forces of I4 engines are not balanced, reducing the overall size they can get to without the use of balancing shafts. Also, I4s have a high center of gravity.