A First Open or FO, is a type of railway carriage used by British Rail and subsequent operators since privatisation. They were first produced as British Railways Mark 1, and subsequently Mark 2, Mark 3, and Mark 4 variants were produced. This type of carriage is an "open coach" because of the arrangement of the seats inside – other types of carriage may be corridor based variants (FK) or have a brake compartment (BFO or BFK).