Pyramid routines use sets at different levels (different reps) of the same weight training exercise to form a pyramid. For example, you start with a set of a lighter weight at 12 reps, then a medium weight for 8 reps, then lift a heavy weight at the top of the pyramid for 4 reps. Then you work back down the other side of the pyramid with a another medium set and then finishing with a set with at a lighter weight.
Pyramid routines are extremely effective for building muscle and perhaps the most widely used workout routines for this purpose. I am of the opinion that these routines should be the focus of weight training programs designed to gain muscle mass.
Full pyramid workout routines aren’t necessarily the most attractive to the weight gainer because they require a lot of time and therefore limit the number of weight training exercises that can be completed in a routine. In practice, most lifters use a forward pyramid or a reverse pyramidroutine. The forward pyramid routine starts with lighter weight and then finish at the top with heavier weight. The reverse pyramid starts at the top with heavier weight and works downward to lighter weight.
By allowing you to train at different levels these routines really stress the muscles well.