If you've read my blog much, you're probably pretty familiar with the hoop (aka Freedom head, Dome, FWB) tree. If you're not familiar with that type of tree, you can read this post to get the basics about them. They're designed to "sit down" on a wide back and a lower, broad wither. On horses with that conformation, a standard tree with an "A" shaped head will perch and be laterally unstable, even if it's the right width. As a result of this low-profile fit, one of the comments I often hear about hoop trees from fitters who aren't familiar with them is, "It doesn't fit. It sits too low in front - there's not enough clearance; I can only get one finger in under the pommel when the rider's up."
Here's a shot of a well-fitting standard tree (note that the angle of the tree point and the angle of the horse's back - both marked in yellow - are pretty much parallel):
There's a good amount of clearance between the underside of the pommel arch and the horse's withers - probably close to the "textbook" 3 fingers' width.
Now, let's look at a hoop tree:
And one with the rider up:
Quite a lot less clearance, right? Note that the tree point angles are parallel to the horse's back, and note that the underside of the pommel arch is clearing the withers. Hoop tree saddles are supposed to fit this way. When fitting a hoop tree, we use the term "adequate" clearance - this means that the balance of the saddle is correct, and that at no time does the saddle come in contact with the withers/spine.
The fitting basics are still the same, no matter what tree type the horse requires. You want The Heavy Seven (plus the billet configuration) to check out ... you just have a little less room under the pommel arch.
I know it may be unfamiliar to some folks, but it's ok. Honest.