Electric bass guitars are obviously a very important part of the sound and that's why a lot of care was put into their amplification. The norm was to connect the guitar to an amp, and then either the amp would send a direct signal to the P/A or a microphone would be placed in front of the amp and then the microphone would get linked to the P/A. A bass monitor was placed on Brian's side of the stage so Brian could hear what John was playing and they'd both be synchronised.
John was, for most of his professional career, a Fender Precision bassist. He was given a 1965 sunburst model by the record company in 1972, and by late 1974 he had another one, which was made in 1968. The two basses are almost identical and they sound 99.99% alike. The only slight difference is on their logos: the 1965 model has a grey logo, the 1968 model has a black logo. John used both of them interchangeably for most of the 70's, and then mostly the 1968 model from then on. Those basses underwent several changes on their finish: in summer 1975, John stripped off his 1965 model and left it natural, one year later he did the same with the 1968 model, and by the time the Magic tour began in June 1986 he had his 1968 bass refurbished and painted black.
From 1972 to early 1977, he took those two Precision's on the road, so whichever he chose for each particular concert would be his main and the other would be a spare in case anything happened such unforeseen damage or serious tuning problems due to an abrupt change of temperature, etc. He also had other Fender Precision's throughout the years:
Fretless (unknown model), used for 1-2 songs in 1977-1979.
Masterbuilt 1955, used briefly in late 1978 - early 1979, then as spare.
Special 1981, used for Under Pressure between 1981 and 1985.
Elite (either 1983 or 1984), used as spare in 1984-1986 plus Freddie's Tribute.
Non-Fender basses included:
Rickenbacker 4001: Used in 1971-1972 (before he got his first Fender).
Music Man Stingray: Main bass for most of early 1977, then used on some songs in 1980-1985.
Kramer DMZ: Spare in 1980-1982.
Giffin Bespoke: Spare at the Tribute concert in 1992.