A carriage is a private four-wheeled vehicle for people and is most commonly horse-drawn. Second-hand private carriages were common public transport, the equivalent of modern cars used as taxis. Carriage suspensions are by leather strapping and, on those made in recent centuries, steel springs. Two-wheeled carriages are informal and usually owner-driven. Coaches are a special category within carriages. They are carriages with four corner posts and a fixed roof. Two-wheeled war chariots and transport vehicles such as four-wheeled wagons and two-wheeled carts were forerunners of carriages.