More Beautiful Sounding Octaves: for the Medium-Size Hand

When I play octaves, there is a tendency, at least in my-sized hand, to have the pinkie and the thumb move towards each other when contact the keys.  But it is worth sometimes practicing in way so that the tip of the pinkie as well as that of the thumb should move in a line along the longitude of their key.  This requires my attention, because the hand is already spread for the octave, and the first and fifth fingers moving slightly towards each other happens naturally.  Especially for the thumb it is a more natural movement.  So, just once in a while, practice octaves so that those fingers move in a plane so that they go directly and horizontally towards the body in an extension of the longitude of their keys.

The muscles needed to move the thumb and pinkie in this direction move in these constrained directions require first, in the case of the right pinkie, an extreme flexion of the third knuckle, down and aimed to the right as it moves in the direction of the body, aided also somewhat by a flexion in the right side of the wrist.  In the case of the right thumb it should practice its motion by slowly tracing  over an imaginary straight line extending beyond the lip the key aimed towards the body.  The third knuckle, where it attaches to the wrist, is prominent in keeping the thumb congruent with this line.  As the motion is made the thumb is always compensating for the desire to move outwards and away from the second finger.