Lock keeper's rota:

Single sited - one lock keeper working at one lock during a working day.
Double sited - one lock keeper working at two locks during a working day. Time during the day will be split between two different locks. Therefore there will be periods of the day when each lock will be on self-service.
Treble sited - one lock keeper working at three locks during a working day. Time during the day will be split between three different locks. Therefore there will be periods of the day when each lock will be on self-service.
Self-service - no lock keeper working at the lock.
ExplanationInformation from 'https://www.gov.uk/guidance/river-thames-lock-keeper-service'