The TRIO is known as the rhythm section. It historically and usually consists of Piano/Keyboards, Bass, and Drums-Percussion.
This fundamental unit provides every bit of snap, pop and crack that you typically associate as a full and comprehensive sound. It is considered the most primary unit, and is often referred to as the “basic” rhythm section.
The Trio can also be Piano/Bass-with vocalist, guitar, violin or horn as the third member.