A collar number, also known as a shoulder number, force identification number (FIN) or occasionally as force number, identifies police officers, police community support officers (PCSO), special constables and some police staff in UK police forces – other law enforcement agencies, such as HM Prison Service, have also adopted identification numbers. Although now displayed on epaulettes, it is still commonly referred to as a collar number. Although most forces issue a collar number to all warranted officers regardless of role, only uniformed officers of the ranks constable and sergeant actually display the numbers.

Sergeant (Sgt) is a rank in use by the armed forces of many countries. It is also a police rank in some police services. The alternative spelling, serjeant, is used in The Rifles and other units that draw their heritage from the British light infantry. Its origin is the Latin serviens, 'one who serves', through the Old French term serjant.