You can longe with just a halter, line and whip, but you won't be getting the full benefit of the exercise and you may be compromising safety as well.
Using halter rings avoids trauma to the mouth, but the halter can be pulled out of position and up into the horse's eye on the far side. Longeing cavessons with rings are designed to fit snugly and stay in place and thus give the handler both more control and more safety than with just a bridle or halter.
Using a surcingle is preferable to bareback because it sends the horse the message of "yes, this is work time, not play time." Surcingles with multiple Ds and rings offer many options for the use of sidereins, breastplate, martingale, crupper and overchecks. A saddle with the addition of girth loops or a grab strap can accomplish some of the same things if the handler wants to ride after longeing. However, a surcingle over the saddle can secure the stirrups and offers even more attachment possibilities.
You may wonder what all those rings are for. Rings on the top, sides or bottom of the cavesson are for attaching the longeline, sidereins, martingale or overcheck. Attaching the longeline is a matter of preference: The top ring of the cavesson keeps the line up and away from the horse's legs, while some feel the bottom ring gives added control. Rings on the surcingle are for the same equipment. They can double as turrets for securing lines when driving from the ground or for longeing with double lines.
Fitting
Longeing cavessons should fit exactly the same as the cavesson on your bridle, making sure it's snug so that it doesn't rotate around. Surcingles sit just behind the withers. A snug fit is essential to prevent slipping around the belly. Padding rolls on either side of the withers form a bridge across the top of the surcingle and prevent rotation.
Some brands of surcingles give a range of fits (for example, 65" to 85"), while others just say "fits most horses." Make sure your horse's barrel fits the size range when ordering from a catalog or visit a tack shop where you can put a tape measure on the surcingle yourself.
Wide belly bands distribute girth pressure over a wider area but, depending on the horse's conformation, may interfere with foreleg movement by coming too close to the elbow crease. Be especially careful fitting thin horses, horses with low withers and horses with more upright shoulders.
