In saddle fitting, choosing a saddle with a wide enough gullet is essential so that the saddle properly clears your horse’s spine. I’ve written about this before — Evaluating Saddle Fit: Gullet width is one of the most popular posts on this site.
Here’s the third in the Saddle Fitting series from Schleese which shows very clearly how to evaluate if a gullet is wide enough for your horse.
The good news is that most newer saddles have wide gullets. Some older saddles may have gullets that are too narrow, so make sure you check that out if you buy an older used saddle.