These first lessons should have no design in training the mouth or the head: first the horse must become supple and acquire facility in turning easily in both directions…
François Robichon de la Guérinière,
School of Horsemanship

RELAXATION is, first of all, a mental state, which in horses is exhibited physically by neuromuscular relaxation. Without this state of mind, no schooling can proceed.
Charles de Kunffy,
Dressage Principles Illuminated

The young horse must already have reached a state of confidence and be familiar with the natural, regular trot. This means that, after it has worked off its initial tenseness, it should trot along unrestrained, without either rushing or being behind the bit, with natural self-carriage on very light rein contact on a straight line.
Waldemar Seunig, The Essence of Horsemanship