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In Anglican church music, a Service is a piece of music for the choir to sing during a service of worship. It uses the words which are a traditional part of the service. The choir is usually accompanied by the organ, but sometimes it can be just for choir without organ.
For Evening Prayer the words will be the Magnificat and the Nunc Dimittis. Singers in the choir often call these "Mag and Nunc" for short. Many composers have set these words. They make them into a piece for choir which is like an anthem.
In the Tudor and early Stuart periods, services were described as "Short", "Great" or "Verse" services. Short services were short, the words were only sung once and it was often unaccompanied (no organ). A Great service was longer: some of the words were often sung many times. William Byrd wrote a famous Great Service. A Verse service was also long: there were several verses which need to be sung by a solo choir member. In between each verse there is some music for the whole choir to sing.