Dumbbell 21’s are an exercise that includes double bicep curling. They’re famously named 21’s because you do three sets of 7 repetitions without resting. They’re unique in that they work the muscle at every strength curve, and you have three. This is why for the first set you curl half way up, second set you curl half way down, and finally for the third you do a full curl. They are intended to overload the muscle in order to promote hypertrophy (increase in muscle size). Try them out!