What is the purpose of long lining a young horse before the first ride and how do you do it correctly?

Long lining — driving a horse from the ground with two long reins attached to the bit or halter while the handler walks behind — is one of the most valuable pre-ride training tools because it teaches the horse to respond to rein pressure from a position that closely mimics the rider's position without placing any weight on the horse's back. Clinton Anderson uses long lining as a bridge between ground work and riding, specifically to confirm rein responses and directional control before a rider is introduced. The horse learns to give to left rein pressure from a handler positioned behind its drive line — which is exactly the position the rider will be in — without the complexity of also managing a rider's weight and movement on its back. The correct sequence for introducing long lining begins with the horse already confirmed in accepting ropes draped over its back, through its legs, and around its hindquarters. The long lines themselves are new equipment, and the horse needs to accept lines touching its sides, hindquarters, and legs before any directing work begins. Anderson runs the lines through the stirrups or side rings of a surcingle to keep them at an appropriate height and prevent them from drooping dangerously low. From behind the horse at a safe distance, the handler asks the horse to walk forward with energy from behind the drive line, then uses light rein contact to guide direction. The horse learns that left rein pressure means turn left, right rein pressure means turn right, and both reins steady means slow down — all from the position and angle that the rider will eventually use. The long lining session should include walking in both directions, simple turns, and halts before the handler considers the horse ready for a rider. A horse that responds correctly and calmly to long lining has demonstrated that it understands rein communication sufficiently to begin the first rides.