Conversation so far:
what does four parts refer to in the musical ensemble?
The four-piece band is the most common configuration in rock and pop music.
who are some examples of a four part band?
The Who, The Monkees, Led Zeppelin, Queen, Ramones, Sex Pistols, Red Hot Chili Peppers, R.E.M., Blur, The Smiths.
what four roles are there?
a vocalist, electric guitarist, bass guitarist, and a drummer.
are there alternatives to that line up?
In some early rock bands, keyboardists were used, performing on piano (e.g.
The Seeds and The Doors) with a guitarist.
what is a hallmark of this type of set up?
Instrumentally, these bands can be considered as trios.
This format is popular with new bands, as there are only two instruments that.

Question: that have what?
Self-contained rewrite: The two instruments have what?