What you should pay attention to when buying a leather leash is the amount of welds and where these welds are located. In a general sense, the fewer welds the better. Having a weld get stuck in your leathers eyes or having a weld from the leash right in your hands is something you want to avoid.
Why are welds in reins?
The leather of reins comes from cowhide. Obviously, a cow has a certain length, and if strips are cut from a cowhide, those strips are 1.5 meters long, for example. If a 2.5-meter leash is then to be made, two strips of skin are needed. Where those two strips are stitched over each other, you get a weld. If that splice is right in your hands, you have that thicker, yet somewhat stiff part of the leash in your hands. You don't want that. That's why you need to buy a size harness. So if you are mentoring a Shetlander, you buy a Shetlander lead, if you have a KWPN you buy a full lead and if you have a pony that is a little bit further in front of the carriage, maybe you should get a cob lead. If you know how long the leash is from the bit to the hand and you know how far your pony is in front of the carriage, you'll figure it out in no time. There are also leathers without welds. Those are much more expensive, because they have to cut them out of a cowhide. Then half a skin is used for one lead and that is of course much more expensive.
Then you have the two-horse leashes. These have to be fully sized, since such a leash splits at some point. Your leash eyes are on the withers, so the split with the buckles should be well behind them. Actually, these buckles should be above the butt of your pony or horse. If you go around the bend and your buckle is against the paddle eye, the horses cannot go around the bend properly because the outside horse does not get enough room. With a two-horse harness, it is therefore very important that you have the right size. Also, you don't want to have the buckle or weld in your hands here either.
You can find more information about types of reins in this blog.
Juliette Post/The Menner