The strength of leather is in the skin side as opposed to the flesh side.  The skin side is the side where the hair was.  When leather is used to make things like saddles and bridles, there is a balance that must be kept between how many stitches it takes to accomplish the job you are doing for the article you are making.  If the holes are too close together, you can weaken the leather.  Too far apart, they aren't strong enough to hold the pieces together.  When repairing leather, we try to reuse as many existing hose as possible to retain the original look of the item as well as the integrity of the leather.  

The picture shows a seat replacement.  By hand sewing everything, all existing holes are being used and no new ones are made, which helps maintain the strength in the original leather. A sewing machine is very difficult to line up to use the existing holes and oftentimes just makes additional holes, thus weakening the leather.  While being quicker, the sewing machine may not be the best way to make a repair.

Adrienne Hendricks is a team member of The Equine Expert LLC, a multi-discipline equine expert witness and consulting firm with expert equestrians offering legal expert witness, consulting services and valuations in court cases and legal matters.  Adrienne is an expert on saddle making, saddle fitting, and specializes in restoration.  Adrienne apprenticed with Master Saddler, Suzie Fletcher, from England.  As a child she grew up riding dressage, combined training, hunter/jumper and was a pony clubber.  She left her profession as a stock broker to follow her passion in the saddle and tack industry.   For more information on Adrienne Hendricks email info@theequineexpert.com.