The USA Equestrian Rule Book defines Reining:
To rein a horse is not only to guide him, but also to control his every movement. The best reined horse should be willingly guided or controlled with little or no apparent resistance and dictated to completely. Any movement on his own must be considered a lack of control.
All deviations from the exact written pattern must be considered a lack of/or temporary loss of control and therefore a fault that must be marked down according to severity of deviation.
After deducting all faults, set here within, against execution of the pattern and the horse’s overall performance, credit should be given for smoothness, finesse, attitude, quickness and authority of performing various maneuvers, while using controlled speed which raises the difficulty level and makes him more exciting and pleasing to watch to an audience.
The official guideline for the application for the rules for judging shall be as specified in the NRHA Judges Guide.