The technique of double notes is the most complex mechanisms executing piano. It is playing successions of two simultaneous sounds with the same hand, often at high speed. For example (Chopin, Study for the thirds, op. 25 n. 6):
The technique of double notes requires a synthesis of all the movements of the mechanism piano: the support of the weight of the arm, the articulation of the fingers, the lateral movement of the hand in relation to the perpendicular of the finger in use. You should also check the relaxation of the arm, forearm and hand, even by small movements of the wrist, elbow and shoulder. At this level of study you must have already acquired sensory awareness of the movements, positions and strategies to prevent stiffening.
To achieve this complexity of goals, in my collection 42 progressive exercises for piano Technique (The technique for the welfare of the pianist) I develop the material in ten exercises, which are precisely designed to progressively lead to the solution of the difficulty.
First we deal with the movement of articulation of the fingers, which represents an improvement to a technique already acquired in the first section of the method.
Then we proceed to the study of double notes, especially the sequences of the thirds and sixths, but also of the fourths and fifths (although less common in the classical repertoire).
A particular attention is then given to the finger passing over on other fingers, similar to what has been studied with the thumb to the study of the scales, but here extended to other fingers, for example: the fourth, third, second finger pass over the fifth, or the fifth passes under the fourth, third, second.
It then proceeds with the study of scales in double notes, and finally an exercise summary, which summarizes not only the technique of double notes, but in the end all the main movements of the piano mechanism: