5.1 Speakers System is a standard AV surround requirement consists of 1 pair of front speakers, 1 centre speaker, 1 pair of rear effect speakers and 1 subwoofer.  Additional matching speakers can be added for an Atmos System.  All the speakers can be floor standing, bookshelf, in-ceiling, in-wall, or on-wall.  The centre speaker should ideally be matching the front speakers as all the dialogues from an AV system come from the centre speaker.