The earliest known type of sidesaddle was a stuffed pillow that was attached to the back of a man's saddle. This was called a "pillion". The lady would have been positioned completely sideways on the horse. There are two short scenes in the movie "The Other Boleyn Sister" of ladies riding pillion.
Some time later a saddle was developed that had a foot rest and back rest. Often they also had hand holds in the front and back. Called a "planchette," this would also place the rider sideways.
It is unknown when the upright horn, the one the rider hooks her right leg around, came into use.
The second horn, an appendage that comes from the right side of the saddle, is commonly attributed to Catherine de Medici. This would cradle the right leg between the two horns and offered a bit more security.
The saddle above is a later version of the "second" horn. Earlier ones were much smaller and morea part of the tree. The leaping head seen in the photo was added recently in the interest of safety. It was not original to this saddle.
Early in this century, the leaping horn was invented. Both the French and English lay claim to this advancement. It was also at this time that the balance strap was created. This strap is attached to the right rear of the saddle, passed under the belly of the horse, and fastened to the left front. This served to stabilize the saddle and offset the extra weight from both legs being on the left side of the horse.
The Victorian era in the late 1800s was during the reign of Queen Victoria and is typically what we think of when we recall "sidesaddle". Queen Victoria wore black for much of her life in memory of her late husband and the ladies of the day emulated her. Could this be the nexus of the traditional black habit seen in the hunt field? A lady's horse was trained to walk and do a collected canter. It was unseemly for a lady to be bouncing at the trot. Riders were quite often sewn into their habits in order to show off their figure to best advantage.
In the 1860s, a western sidesaddle was attributed to Charles Goodnight from the United States. He had a saddle made for his wife, built on a western-style tree. It was believed that the typical English saddle was not sturdy enough for the rough country out west. There are photos of Belle Starr (a notorious bandit and horse thief) on her sidesaddle, usually with her six-gun on her hip. Her saddle is believed to pre-date the Goodnight saddle.
The western sidesaddle often included a "purse" on the offside to hold the lady's incidentals such as coins for tolls, a handkerchief, and, possibly, a small deringer for protection.
Most of the saddles used today are holdovers from the late 1800s through the early 1900s. Four manufacturers are desirable for a quality ride. These are Mayhew, Owen, Martin and Martin, and Champion and Wilton. New saddles are being produced but are quite pricey and are best left to the serious sidesaddle devotee.
BEWARE of buying a saddle online! Many of these saddles are of poor quality materials and construction. A badly fitting saddle will be uncomfortable for both you and your horse. There are sources for learning about fit and what to avoid on the NEA Sidesaddle Association website.
Some safety features are considered a necessity:
A safety stirrup ... one that breaks away at the foot rest or one that the leather separates from the saddle in case of a fall.
A safety apron ... a skirt-like garment that has much less fabric than a regular skirt to get caught on the horns.
Some riders like to carry a sidesaddle cane to cue the horse on the right. A dressage whip (longer than a hunt crop) can also be used.
With the exception of roping events, sidesaddle riders enjoy nearly all types and styles of riding. There have been ladies riding aside in eventing, dressage, endurance, barrel racing, fox hunting, and most types of pleasure events at shows.
Sidesaddles are beginning to be used in handicapped riding programs. As was the case after the wars, especially WWI, riders who for any reason cannot ride aside, can enjoy their horses sidesaddle.
Some famous ladies who rode sidesaddle include Queen Elizabeth I, Queen Victoria, and today's Queen Elizabeth II. Other riders include Laura Ingles Wilder of "Little House on the Prairie" fame, Annie Oakley, and Sybil Luddington, who rode to warn of the British coming just like Paul Revere. She rode furter and she didn't get caught!