First, we need a horse to fit - so, meet "Excitable Boy", aka "EB".
EB is an off-the-track Thoroughbred, 9 years old, who's spent the last 4 months in a steady program of dressage. He's a bit rump-high, with a decent wither and a somewhat dropped back. So, in addition to a tree that's the correct width, we need a tree that has some curve or "scoop" front-to-back. I'm going to show two trees below, and then show how they'd fit on EB.
Here's the Black Country Eden tree:
And here's the Black Country Eloquence tree:
I PhotoShopped the two trees so they're one solid color, and removed the background on EB's photo - makes things easier to see - and put the trees on EB. Here's the Eden tree:
And here's the Eloquence tree:
If you look closely, you'll see that the Eloquence tree, which is quite flat, bridges slightly. While a panel modification could make up for some of that, it wouldn't be ideal. The Eden tree, which has more scoop, is a much better fit.
Now, let's take it to the other extreme. Here's Remmy, who's pretty flat front-to-back:
Now here's Remmie with the Eden. I had one heck of a time getting the tree to sit correctly - even in PhotoShop! If I made it sit level, the cantle popped up:
And if I made the tree have proper cantle contact, it sat pommel-high:
So the tree of the Eden is too curved, and would make the saddle rock back-to-front.
Now here's the Eloquence:
The flatter tree sits with good contact along its entire length, making it a far better choice for this flat-backed boy.
In my next entry, I'll be covering the next component of the saddle: the panels.