The SM postcode area, also known as the Sutton postcode area, is a group of seven postcode districts in England, within five post towns. These cover part of south-west London, as well as a small part of north Surrey. The main sorting office is in Sutton, with outgoing mail being sorted at Croydon Mail Centre.
This trialled the postcode area CRO (letter 'O'), while Norwich used NOR for its equivalent. It was decided that the outward (first) part of a postcode should be alphanumerical, so CRO became CR0 (with a zero) and NOR became NR1. The CR1 postcode has not been introduced, making CR the only postcode area with a district 0 but not a district 1.