usually a higher wither and wider clearance at the shoulders. Gaited trees tend to flair at the front, and have alot of wither clearance. Probably has more to do with giving the shoulders clearance while they are moving then an actual shape thing on the standing still horse. Depending on the variety many are also short backed. I'm not sold on treeless designs. I spent to many years carrying rucksacks, carrying anytype of weight without a frame blows. They are probably fine for people 100lbs lighter than me. I'm not interested in trying one. Also with the popularity of Arabs in the sport I imagine many of the saddles are made for Arab backs. Never owned one, so not sure how they compare. I just see saddles advertised with Arab trees as opposed to other trees so I assume something is different.