Second Open or Standard Open (coded SO) is a British coach designation for open-plan second class (later standard class) coaches with 2+1 abreast seating, usually seating 48 passengers. Second class coaches with more conventional 2+2 abreast seating were designated Tourist Standard Open (TSO).There were a number of variations. The majority (95) were built using the same body shell as the Mark 1 SK and TSO. When production started, these were classified as Third class.