Problems in how the horse is saddled, or other padding and tack: Only by watching the rider saddle up can things like saddle and cinch position and tightness, blanket quality, condition and placing, and breast collar or crupper placement be properly assessed to see if they contribute to the fit problem. 
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1)  Saddling:
a)  Position:  Given the option, a tree will move to where its shape best fits the shape of the horse’s back.  The problem arises when the rider thinks it should be somewhere else, and not only starts the saddle in the wrong place but holds it there with a breast collar or crupper.  The saddle is often placed too far forward, interfering with and even being forced to ride on top of the shoulder blades.  Placing a saddle too far forward may cause the cinch to rub behind the elbow.  Other riders are so concerned about shoulder interference that they place the saddle too far back.  A good fitting saddle will cause problems if forced to remain in the wrong position.  If it is not held there, it can cause skin irritation or wrinkles in the blankets as it moves into the correct position.  The best way to saddle is to place the saddle one to two inches ahead of where it should sit and then shake it slightly side to side.  This way it slides back, with the hair, into the correct position, where it can then be tightened.  Some riders need to step back and be shown where the saddle is meant to sit relative to the withers and the front legs in order for them to understand how far out of position they are placing it.  Tact is required in this instance.
b)  Cinching up:  A good fitting tree should only be as tight as needed to keep it in place when mounting and riding properly.  For roping or doing fast work, it will need to be snugged up more.  But sometimes the saddle is cinched up tight and then ridden or left for long time periods without being loosened.  In this case, both the cinch and the saddle are exerting unnecessary, excessive pressure on the horse and in the long run may cause problems.  The old cowboys not only “aired out their horses’ backs” during the day by uncinching and lifting the back of the saddle, but often unsaddled over lunchtime to give their horse’s back a break.  Cinching too tightly for too long will exacerbate any potential problems areas and may even cause the problems.

2)  Padding:
a)  Type:  When you ask people who do ranch work on horses for long hours, day in and day out what type of pads they use, the answer is close to unanimous – wool or wool felt.  Wool dissipates pressure better than any other material, and it conforms to the shape of the horse’s back better than anything else.  It doesn’t trap heat next to the body like a lot of synthetics do.  Wool pads are not slippery, as some synthetics are, so they stay put and help hold the saddle in place with less cinch pressure.  Pads contoured to fit over the withers don’t put pressure on the top of the withers like unshaped pads do.  This pressure, if unrelieved over a period of time, can harm a horse. 
b)  Amount:  Over padding is a common cause of saddle fit problems.  Too much padding negates the good fit between the shape of the horse and the tree by diminishing the curves.  It causes the saddle to roll around more so the cinch needs to be tighter to hold it in place, causing possible cinch soreness problems.  It also effectively widens the horse, often causing dry spots and maybe necessitating a wider, poorer fitting tree to make up for the added width of the padding. 
c)  Condition:  Dirty blankets, folds or wrinkles - anything that rubs on a horse can cause sores.  Worn or damaged padding that is no longer a uniform thickness may also cause problems. 
d)  “Corrective”:  Shims, wedge pads, pads that have varying thicknesses in different areas – all these have to be used with caution or they may cause more problems than they solve.  Thick edges are especially bad for making ridges that cause pressure points. 

3)  Other tack:
a)  Breast collars and cruppers:  A good fitting saddle should rarely need either of these.  Used incorrectly, they often cause problems by holding a saddle in the wrong position.  Improperly adjusted, they can rub and even sore a horse. 
b)  Cinches and latigos:  Synthetic cinches and latigos have no give to them, so using only synthetics all the way round makes it more difficult for a horse to expand its chest when breathing.  As well, there is no stretching or loosening up as the horse warms up to give relief from tight cinching.  It is recommended that at least one section be made of natural materials to avoid these problems.  The narrower the cinch, the less surface area the pressure is spread over.  While some cinches appear to be wide, all the tension is actually put on a narrow band of nylon that runs down the middle of it.  Some synthetic cinches also hold a lot of heat next to the horse.
c)  Uneven loading:  All manner of things are tied onto saddles or carried in saddle bags.  While each item may not weigh much individually, putting all the fencing tools in one saddle bag or continually having a heavy water bottle on one side will weight the saddle unevenly, causing constant shifting to one side and increased pressure on that side. 
  

1.)  Behavioural problems in a horse that anticipates or reacts to pain from the saddle. Behaviour problems can be a problem with the saddle, a problem with the work the horse is asked to do in that saddle or a problem with the attitude of the horse and these causes need to be distinguished. 

•      Reacting negatively to being saddled.
•      Behaving badly under saddle, especially with a particular saddle
•      Avoiding pressure when rope comes tight

2.)  Changes in movement or ability to perform compared to other saddles or when unsaddled.  These can include:

•      Shortened stride
•      Poor collection when asked
•      Not wanting to stop hard

3.)  Instability of the saddle.

•      Constantly rolls side to side
•      Consistently rides to one side
•      Loses blankets easily
•      Moves forward or back on the horse constantly. Doesn’t find one place and stay there.

4.)  Physical problems on the horse

•      Sore muscles on palpation (checking by feeling with moderate pressure)
•      Swelling which develops a few minutes after unsaddling
•      Dry spots  (Whether these are always a problem is open for discussion, but they are at least a signal to check for deeper problems.)
•      White hair
•      Open sores (usually caused by rubbing rather than pressure)
•      Pain or sores on top of withers from direct contact with the saddle

If the shape of the bars match the shape of the horse’s back, the fit is like two spoons nestled together.  Unless it is held in the wrong place by a breast collar or crupper, the tree will move to sit where it fits properly and then stay there.  Riders tend to be overly concerned about rigging position being too far back so the latigos and cinch angle forward.  It may slant forward more than what the owner would prefer, which may make them think the saddle is too far back.  Some even move the saddle forward and hold it out of place with a breast collar to make the cinch stay vertical, causing saddle fit problems.  We maintain that if the tree fits the shape of the horse’s back, it doesn’t matter.  The rigging position won’t pull the saddle out of place if it fits well, even at center fire.  If it is a poorly fitting saddle, then there is no “curve-and-hollow” fit stopping the rigging from moving the saddle around.  If the saddle continually moves forward onto the shoulders so the cinch is vertical, it doesn’t fit well. 

A saddle rigged very forward on the bars (full or more) will pull down much more on the front of the bars, often impeding the shoulder blades from slipping under the bar tips as they should do when the leg is fully extended.  The same saddle with a rigging farther back allows much freer shoulder movement.  This goes counter to the “rig it forward to hold the saddle back so you get free shoulder movement” idea, but it has been shown to be a fact.  If there are problems at the back of the shoulders and the saddle is rigged full, moving the rigging back may solve the problem.
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A saddle that is double rigged is meant to have the cinch doing something, not just hanging in the air waiting to cause a wreck.  However, the back cinch will loosen up over the course of a ride.  Constructing the rigging so that it exerts its pull more centrally on the tree is a good thing.  For saddles rigged with the front cinch pulling mainly on the front of the saddle, doing the front cinch up tight increases pressure under the front bar pads and lifts the tree off at the back.  If the rider is not willing to continually tighten the back cinch to the point of contacting the horse, it is better to change the rigging so it will have a more central pull on the tree. 

The basic and first principle of saddle fit is to “Position the Saddle” in the place it was built to fit.  This is the responsibility of the rider, and neither the saddle or tree maker can influence where the rider decides to place it.  The traditional Western tree is built to fit right behind the shoulder blade and shaped so that as the horse’s leg is moved forward, the back corner of the shoulder blade will slip under the rounded front edge of the bar.  By the time the horse puts weight on that leg again, it is back out from under the bar tip.  This means the front of the skirts (generally less than 2” past the bar) will be over the shoulder blade, but if they are properly blocked (shaped to follow the curve of the bar), they will not affect the horse’s movement.  The idea that you need to place the tree a couple inches behind the shoulder blade to avoid hitting it is false because the bars are shaped to account for that.  However, even if it is improperly placed, the saddle will very quickly move to where it belongs unless

1) it really doesn’t fit the horse at all, or

2) it is held out of place by a breast collar, crupper, etc. or

3) it is placed so far ahead on the horse there is no reason for it to move back into place.

​Placing the saddle too far forward so that it rests on the shoulder blades and then holding it there with a breast collar will negate all the work the saddle and tree maker put into making a saddle that fits the shape of the horse’s back.  

The idea that you need to put the rigging forward on the saddle so it hangs vertical in the "girth groove" of the horse or it will pull the saddle out of position is a fallacy. The position of the saddle on the horse is determined by the shape of the tree matching the shape of the horse, not the rigging position or cinch placement.  

The cinch will move to the narrowest part of the chest (unless held away from it by something like a center fire rigging position).  But an angled latigo and cinch will not pull the saddle out of position if the tree fits.  It is only if the tree really doesn't fit the horse or there is excess padding which negates the fit of the tree on the horse that the rigging position can have an effect on saddle position.  Rod has used his 3/4 rigged saddle (pictured above) on many different horses over the past almost 25 years.  The only time the saddle has moved forward was on a couple really wide, round backed horses it was too narrow for.  On one "interesting" backed horse he used a lot, he actually needed a breast collar to keep it forward as it tended to slide back on that horse, even with the latigo angled forward.
It may also appear that a latigo and cinch that is angled forward would cause the saddle to put excess pressure on the back of the shoulder blades, while a saddle with a full rigging position (which is most likely to have a vertical cinch) would be least likely to affect the shoulders.  However, this is backwards from what has been shown by pressure testing (unpublished data) to happen in real life.  The more forward a rigging is set, the more it pulls down on the front of the tree.  The farther back it is set, the more it can pull down more evenly on the whole tree.  Of course, rigging design and construction has a large effect on how and where the rigging pulls down on the tree, but practically, a rigging set below the fork will pull down almost exclusively on the front of the bars.  Therefore, a forward rigging position can actually cause pressure points at the back of the shoulder blades because it pulls the saddle down tightly at the front and so it doesn't give the shoulder blades enough room to slip underneath the front bar tips so they "bang against the wall".  The same saddle with the rigging moved back could allow free shoulder movement.  A rigging that is set farther back will probably end up with a latigo that is angled forward, but it won't pull the saddle forward and it will free up the shoulders more than a forward set rigging.

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The Bare Basics Summarized

1.)  Saddle position is determined by the tree, ending up where shape of the tree matches the shape of the horse.
2.)  Western saddle trees are made to sit right behind the shoulder blade, which puts the skirts just over the back of the shoulder.
3.)  If the tree fits at all, the saddle will move to the correct position unless you keep it out of position with a breast collar, crupper or britching.
4.)  Rigging position does not determine or change saddle position unless the tree doesn't fit the horse or over padding causes the tree to no longer match the shape of the horse.
5.)  Rigging position will not move a good fitting tree out of position.
6.)  Forward rigging positions can impede shoulder movement.
7.)  The skirts need to be well blocked to follow the relief built into the bars so they don't interfere with the shoulder blade.