The straight-4 or inline-4 engine (often abbreviated I4 or L4) is a four cylinder internal combustion engine with all four cylinders mounted in a straight line along the crankcase. The single bank of cylinders may be oriented in either a vertical or an inclined plane with all the pistons driving a common crankshaft. Where it is inclined, it is sometimes called a slant-4. The straight-4 layout confers a degree of mechanical simplicity which makes it popular for economy cars.