First check leathers and be sure that the rider is not one sided – because that will wear the saddle more on one side, and thereafter, without, expert flocking to get it level again, it will sit to one side. It may be an inappropriate saddle – either too narrow or too wide and, very occasionally these days, it could be the tree- but it may also be a bio/mechanical issue If it can clearly be seen that the horse is made up more on one side than the other and that correct work in a correct outline is unlikely to improve the problem you may decide to ask your saddler to flock the saddle to sit straight on the horse. If you decide on this – YOU MUST NEVER USE THIS SADDLE ON ANOTHER HORSE. and it must be checked regularly. In view of the enormous number of correctional pads on the market, at the NSC we prefer to advise the use of something appropriate adding extra shims wherever needed and keep the saddle as straight as we can.
In the last few years we have had many advocates for the "Wide saddle”, - wider than the horse warrants, and the use of various front riser pads, saddle cloths , shims etc. to get acceptable balance. Looking at this from the viewpoint of building muscle this would appear to be the right way forward and there are cases where with educated riders, very conscious of their ability to sit straight and keep their horse straight that this approach can work extremely well – However, in practice, it is not always successful - horses like riders have a “jelly side” and we are nearly always stronger on one side than the other. It is very easy in such circumstances to find a saddle which, in every other respect is excellent, lurching off to one side.