When practicing the following exercises be careful that both fingers strike the keys at the same time and with equal force. Play from the knuckle joints, and raise your fingers as high as possible. These exercises should be played first with each hand alone, then both hands together. Play them first slow and then fast.
Listen very carefully to your playing and persevere until you can play each number smoothly and rapidly.
Etude for practicing thirds on piano
Lift your hands from the keys during the rests. Play in the legato style, and count carefully. This Etude is for the purpose of practicing runs in thirds. Play slow and smoothly.