The starting rotation is an interesting and revered construct in baseball. It usually consists of a group of five pitchers who are collectively a unit but do not really play together. Every five days, a new pitcher steps onto the mound and his comrades follow in the four days in between.
Building a rotation is predicated on picking pitchers with a combination of effective stuff and enduring stamina. There have, of course, been experiments with the starting rotation model, like when the Tampa Bay Rays instituted the opener. However, for decades, the five-man starting rotation has served as a staple.