"In the tradition of the old Spanish Riding School, three repetitions of the same exercise were defined as a reprise. That is a good structuring device, because you start to see a trend after three repetitions. The horse's gait and posture will either improve or deteriorate.
If you ride something once, the outcome could be a coincidence.
If you get the same result three times in a row, it's a pattern.
If there is no improvement within the first three attempts, it's unlikely that things will get better during the next hundred repetitions.
Therefore, you should then modify the aids or the exercise, or in extreme cases abandon the exercise completely for the time being.
After the first reprise, you change direction and ride a reprise on the other rein. Then you compare in which direction the exercise was more difficult for the horse, and in which direction it benefited the horse more.
In the past it was customary to ride a third reprise in the more difficult direction. This protocol prevents mindless drilling and thoughtlessly repeating the same mistake over and over."