. The following is part of an article I found, on harnessing a horse, writen by Bill Morong, and dedicated to "trying to help modern drivers and their horses enjoy driving safely and comfortably."
It is a very good article and hopefully you will take the time to read the entire article, here is a link. This is also a very nice harness shop. Camptown Harness - camptownharness.com and horseharness.com - 800-717-0957
Part 3 - The Vehicle Support And Steering System
The main component of the vehicle support and steering system is the saddle (31) or pad. The saddle is essentially a wide, padded, stiffened strap over the horse's back. The saddle comprises a tree, to which flaps (31) are affixed, topped by a skirt (33), and supported on padded panels. The saddle, if black, may have patent-leather skirts, welts, and pug seat. Flaps may be patent, but patent flaps lack durability. For russet saddles, pigskin is proper wherever patent leather would be used on a black saddle. The saddle is fitted with terrets (34), which guide the reins, a waterhook (35), which anchors the bearing rein, and to its rear a crupper-strap staple, and it may be fitted with a purely decorative pug seat.
The horse steers the vehicle by pushing left or right into the saddle. For two-wheeled vehicles the horse, through the saddle, supports the front of the vehicle, in which case the saddle must be wide enough to distribute the weight transmitted through the shafts. The saddle must not, however be so wide as to prevent proper ventilation of the horse's back. About four inches is average for a thousand-pound horse with a two-wheeled vehicle. For four-wheeled vehicles, the horse bears only the weight of the shafts, and the pad can be narrower. The saddle is stiffened by a tree. Trees may be rigid, spring, or soft. Rigid trees must fit, spring trees ought to fit, but are usually too rigid to conform, and soft trees allow the saddle to become misshapen if incorrectly stored. A rigid tree must be formed to fit the angle of the horse's back.
The underside of the saddle is padded. Sufficient padding must be installed to lift the gullet of the saddle above the withers so that with the full vehicle weight applied a finger will fit above the bony processes of the vertebrae. Sometimes the padding is contoured to fit the horse. The padding may be quilted, which causes it to grip the hair of the horse and allows a looser girth. The finer the quilting, the greater the grip. Decorative tufts are sometimes applied, which are harmless along the ballasts, but should not be applied where they could irritate the horse's skin, as tufts are difficult to clean and tend to harden.
There are two styles of saddle flaps, tapered straight-sided, and swelled, but their selection is dictated by a consideration more important than mere taste. Straight-sided flaps are usual, and usually retains the shaft-tug best. Some horses have withers swelling forward like the mouth of a trumpet, and ribcages swelling rearward in the same manner. With this conformation, unless the saddle flaps can be twisted nearly ninety degrees in a few inches, the rear edges of the flaps will dig into the ribcage. Swelled flaps are needed to provide the needed flexibility for this situation.
The saddle is retained by a girth (38). The girth may be padded, but is better made plain, with a slippery-when-wet inner surface. If both saddle and girth grip strongly, chafing may occur in distance driving. The width of these parts is a compromise between good distribution of forces and good cooling. On some horses the girth tends to ride forward, causing girth sores. A girth in the form of a low isoceles triangle can be acted upon by the pectoral muscles, driving the girth back out of harm's way.
The single horse saddle is provided with shaft tugs (36) to transmit the force of shafts to the saddle. There are three types of shaft tugs: English(or open), French, and Tilbury. English tugs are common with straight-shafted, two-wheeled vehicles, and gigs. With English tugs, if shaft stops are fitted, examine the slots of the heads of the screws attaching the shaft stops for burrs, as such eat English tugs. French tugs are used with two-wheeled vehicles with upward curving shafts, and when the vehicle is incapable of being balanced. French tugs are often considered to be fancy, but the are very practical wherever the shafts must be held down, as would usually be done with English tugs and a wrap-girth. Since they are both fancy and practical, French tugs are very versatile. Watch out for tacks and staples attaching leather shaft covers as they eat the leather lining of French tugs. Tilbury tugs are for four-wheeled vehicles.
Vehicles should be balanced never to apply more than momentary upward shaft force, but to avoid more than ten or so pounds of downward pressure on each shaft. Balance is obtained by moving weight fore and aft of the axle, usually by sliding the seat. Changes in the number of passengers may require changes in balance. With English tugs, ideal balance is indicated when the shafts begin to float in the tugs when climbing a steep hill. The tugs may be supported by a running backband (32) which slides from side to side in a channel inside the saddle. This arrangement allows the horse and saddle to rotate within the shafts as the horse moves, which is easier on both horse and passengers. A running backband must never be used on a four-wheeled vehicle with articulated shafts, or one shaft may rise and the other fall -- an embarrassing predicament.
A saddle with a running backband will have its terrets set far forward of its centerline. If a running backband is not used, bearer dees emerge directly below the saddle skirts. These dees are affixed by terrets centered on the saddle and by pad screws on both skirts. These dees support bearer billets to which the shaft tugs are buckled. The level of the shafts is adjusted by buckling the tugs up or down on the backband or bearers. Shafts (51) too high can cause a shaft over the neck causing an accident. If the shafts must steeply angle up toward the horse to be the right height at the horse, the vehicle is not high enough. Ideally, straight shafts are level and aim toward the place where a breast collar should lie on the horse's breast. Ideally, the shaft tugs should not rub on the girth billets (37), but since most saddle flaps are too short, rubbing often occurs.
Shafts must be securely held down by some type of shaft girth (39), which must be carefully examined, as its failure is very dangerous. With a running backband and English tugs, the backband often extends down past the shaft tugs to a shaft girth that runs in loops on the saddle girth. These loops must always retain the path of the shaft girth upon and below the saddle girth even when descending steep hills, or the horse's skin may be pinched between the girths.
It is often advantageous to couple the vehicle more tightly to the horse than is possible with the traditional arrangement of English tugs, in which case a wrap girth may replace the shaft girth. The wrap girth is wound around the shafts and pulls them in and down, which gives tightened control around obstacles. If the use of a wrap girth on a saddle with a running backband is desired, a shorter backband is fitted. Some English tugs are fitted with dees and billets at their bottoms. Examine these tugs carefully for cracks and wear by thinning where the dees pass through the tug bottoms.
French tugs are fitted with billets that hold down the shafts and attach to a shaft girth. Tilbury tugs function similarly to French tugs, but are appropriate for use with four-wheeled vehicles only.
Some racing or roadster saddles are fitted with shaft pockets or thimbles that fit over the shaft tips to transmit breaking force to the shaft-tug bearer dees. This system of braking is lightweight and works on flat ground, but is not appropriate for carriage driving, as it pushes the saddle forward when braking, and would soon sore a horse in hilly terrain. Similarly, shaft stops must be located and holdback straps adjusted so that the shaft stops do not act during braking before the breeching can transmit the force to the buttocks or chafing around the saddle may occur.
First is a photo of The pieces of the saddle.
There is the padded saddle with a tree, flap and a skirt.
the backband connects to the shaft tugs on each side and then to the outer girth, and the saddle flaps connect to the inner girth.