The four piece band was the early rock and roll success formula. Lead Singer, guitar, bass and drums was the standard outfit, often with a lead singer playing rhythm guitar and/or harmonica. Shortly after rock and roll started to make an impact a fifth person on keyboards was added, sometimes replacing the need for a rhythm guitar.
In the beginning the formats were depending on 2 factors:
The financial split and the stage space available in clubs and venues. In order to perfect their chops most starting bands went “on the circuit”. In the southern US that was called the Chitlin’ Circuit. In Europe it was called the rock and roll circuit. Geographically calibrated, the venues offered weekly entertainment and the bands made sure to build an audience for the next go-around usually 8 to 10 weeks later.
Many of the early rock and blues venues had no entertainment budget and loosely allocated a percentage of the tap for the band. Obviously smaller outfits had a better share. The better and more all around the guitarist was, the less members the band needed. Also the space allotted for equipment was a major factor. Bands needing huge equipment space were in lesser demand. A typical back line was amps and speaker box for each of the guitars (sometimes lead and rhythm guitar doubled up on one amplifier), standard drum kit, and a voice amplifier with echo abilities for the voices. No PA systems or Mixers. No FX pedal boards for guitars. One distortion pedal and after 1967 a Wah-Wah and that was basically it. Keyboards were of the 61 key type version and much later were taken up to 88 keys. Space was limited.