The gullet of a saddle is the channel that runs down the center of the underside of a saddle, in between the panels. The gullet bridges the horse’s spine so it is an important part of the saddle design. 
If the gullet is narrow, it can pinch the muscles on either side of the spine which ultimately will make your horse’s back stiff or sore. The horse’s spine is where the nerves of the back are located; to prevent the saddle from impinging on these nerves, the gullet needs to be wide enough to distribute the weight of the rider on the horse’s muscle.
Many people confuse Tree Size with Gullet Width. Tree is a completely different measurement: it refers to the distance between the bottom of the points of the tree. This measurement is either described in centimeters or as medium, medium/wide, etc. Please note that these excellent descriptive photos are from: How to Measure an English Saddle. The rest of the photos are mine, but these just illustrated the concepts more clearly.
I think that part of the confusion between tree size and gullet width is due to the popularity of the Wintec saddles with interchangeable gullets. The concept promotes the idea that by interchanging the “gullets” you can adjust a saddle from a narrow tree size to an extra wide. And you can. But the Wintec gullets are inserted into the front of the saddle and they change the angle of the tree — and may change the front part of the gullet slightly. If you look at the photo on the left, imagine that the inverted “V” is either narrower or wider. In some saddles, the saddle gullet is wider at the pommel end and narrows over the length of the saddle.
In fact, there are saddles where the gullet becomes very narrow indeed. In the photo below are two saddles: an old Hermes close contact saddle and a recent model County cross country saddle (the Extreme). 
They are both medium trees, according to their manufacturers, but they are different sizes. The Hermes is a 16.5″ and the County is an 18″.
As you can see, on the Hermes saddle, the gullet narrows significantly as it approaches the back of the saddle.
In fact, you can barely fit two fingers in the gullet of the Hermes.
In comparison, the County has a nice wide gullet. In fact, you can comfortably fit three fingers in the gullet. I’ve read that the “three finger” rule is a good starting point for gullet width, although each horse is different.
The implications of this difference comes when you look at the saddle on a horse. The model here is my 17 year old Trakehner gelding, Kronefurst. The first photo shows the Hermes.
You can see that the gullet offers barely any clearance over the spine and the panels sit very close to the spine and will not distribute a rider’s weight effectively.
In comparison, look how nicely the County clears Kroni’s spine. 
The panels sit very nicely on his back and will distribute a rider’s weight very well. And they should! I have my saddles checked and fitted by a professional saddle fitter at least once a year.
Given that narrow gullets can impinge on a horse’s spine, why do manufacturer’s make them this way? One rational for making the gullet narrow is for rider comfort. Many people like riding in a saddle with a narrow “twist”, and some saddle manufacturers simply bring the panels closer together toward the back of the saddle to accomplish this. However, it’s not nearly so comfortable for your horse!
Can a gullet be too wide? Yes, if it reduces the size of the panels to the point where they can no longer adequately distribute the rider’s weight. This is a fairly unusual; it is more typical for gullets to be too narrow.
Thank you for this informative entry. I’ve been in the industry for sixteen years now, and I cannot say that I have read a better explanation. You did a great job illustrating the difference between tree size and gullet space. Also, the pictures from the rear of the horse emphasize the need for fitting the saddle to the horse not the other way around. Thank you again for this interesting and informative article.
I also want to thank you I have been doing some research and I couldn’t find anything on this except the easy change gullet system. I have been searching for something to help me to determine whether my saddle fit both my horses and if I need to get a new saddle. I do have a question how ever so I was wondering if you could help me? I have one horse that is a paint Thoroughbred cross but he is very narrow in his shoulders and down his spine and my saddle seems to fit him good but my other gelding who is also a paint has very wide shoulders with a high whither bone and the saddle is constantly sliding back so I don’t know what I can do to fix it I have tried a breast plate but it still slides back and the breast plate ends up restricting him. So can you please help me any advise is welcome.
Thanks again
Hi Katie, thanks for reading EquineInk. I’m glad the article was helpful. Without seeing a picture of the saddle on your paint/TB cross it’s hard to say exactly what’s happening but it sounds as if the tree is too wide for him. Most of the time when a saddle slips to the extent that you describe, that is the cause. Depending on how much too wide the saddle is, you may be able to help the fit by using a pad that has shims in it to build up the front part of the saddle. Two manufacturers who make them are Mattes and Thinline. Just be careful that you don’t cause different pressure points by adding padding. I’m going to write up a post on the pads soon.
Liz
Pingback: Dressage Maven » Weekend Link RoundUp - Saddle Fitting
Pingback: Tips for Buying a Used Saddle « EQUINE Ink
Pingback: Saddle Fitting, Part II: Gullet Width « EQUINE Ink
Thanks so much for this great explanation. I just got a new (well, new to me saddle) today and I was looking for information about how to tell if it will fit my horses. Your information is very clear! I appreciate the pictures!
I’ve been through heaps of webites, seeking the answere to the great mystery of saddle fitting!!! Simply wanting to measure my gullet and it seemed like the hardest question to get a answere for. Thank goodness I found your website, you explained everything perfectly and now i can confidently ensure my saddles fit my horses, even though i will still get inn a saddle fitter to double check. Thanks so much
Glad to help!
Pingback: A year of blogging at Equine Ink | EQUINE Ink
do you have any recommendations for high wither saddles. thanks
great article- =)
Thank you!
Thank you!!
Very informative and educational article!! I’ve been in the industry for over 20 years and I’ve never read something so clear and understanding!!! Great pictures for viewing. You should do demos at the Horse World Expo here in Harrisburg, PA when they are here in February every year. Thank you so much,
Kelly and Doug
Thanks! And welcome to Equine Ink.
Thanks for the help on gullet width. I learned alot and will be more careful when I am trying out a new saddle to make sure it has clearance of the spine
.
Your a life saver! it’s so hard to buy a used saddle when you don’t know what size it is!