A Brief Definition of “Frame”

The horse’s frame is the length of the animal’s body as controlled by the rider. When a rider shortens his horse’s body and length of step, he is collecting the horse into a “shorter frame”; when he elongates the body and length of step, he is lengthening the horse into a “longer frame.” A horse traveling on loose reins has “no frame.”

The rider has collected the horse into a short frame for the working trot sitting.
This horse is shown in a long frame during a Hunter Under Saddle class.
The horse has "no frame" as it trots, which is commonly called being "strung out."