Start with the right size, measured not guessed
Rug sizes run in feet and inches measured along the horse, so run a tape along his side — starting at the middle of his chest, passing round the widest part of his shoulder and barrel, and ending at the point of his hindquarters where you want the rug to finish. Buy to that measurement rather than to what his stablemate wears. A size too small pulls back off the withers, presses on the chest and rubs the shoulders raw; a size too large slides sideways, hangs off the quarters and twists until leg straps are where they should never be. Between sizes, the deciding test is the front: the rug should close at the chest without gaping and without straining, with the top edge sitting in front of the withers rather than on top of them.
Put it on the way it wants to settle
Throw the rug well forward, further up the neck than it will finally sit, and let it slide back into place as you fasten it; dragging a rug backwards into position ruffles the coat against its lie and starts rubs before the horse has moved. Fasten the chest straps first, since they anchor everything else, then cross the surcingles under the belly, adjusting them so you can slide a flat hand comfortably between strap and skin: tighter risks pressure and a caught foot when he lies down, looser lets a leg through when he rolls. Thread each leg strap through the other before clipping it to its own side, which keeps them centred and off the insides of his legs. Stand back and look: the rug should lie level, the seam along the spine straight, and the front clear of the windpipe when his head comes down to graze.
The rub points, and how to check them
Rugs rub where horse and fabric move against each other: the points of the shoulders are the classic site, then the withers on high-withered horses, the chest where straps sit, the hips on fine-skinned animals and along the mane. Check daily while he is rugged, because a rub found at the broken-hair stage is a fitting tweak, while a rub found at the sore stage is days off and a rug off. Run your palms over those points feeling for heat, raised skin or tenderness, and part the hair to look for short, broken or whitening hairs. Reach under the rug at the shoulder and withers to confirm there is no tight ridge of fabric pressed into the skin. A silky shoulder guard or bib under the rug cures most shoulder rubs without changing rugs, and a rug with a better-shaped shoulder dart is the longer-term answer for a horse who rubs every winter.
Fit changes with the horse
A rug fitted in autumn is fitted to an autumn horse. Through the winter he may gain or lose condition, grow or shed coat, and be clipped, each of which shifts how the same rug sits. Recheck the whole arrangement after any clip, after a visible change in his weight, and whenever a new rug replaces an old one, even at the same labelled size, because cuts differ between makers. Watch how the rug behaves in the field too: one that has twisted round by morning, or whose horse comes in sweating under it on a mild day, is telling you the fit or the weight is wrong for him now.
Checking is a shared job, so share the details
On most yards the horse is rugged by whoever gets there first, which means the fit you achieved on Monday can drift by Friday through no one's fault. A short written note solves it: his size, which holes the chest straps use, how the surcingles are set, that his left shoulder rubs by late winter and needs the guard, and that he is checked under the rug morning and evening. Family Sharing on KinsLe is how his profile reaches the relatives you pick, and nobody outside the family you choose gets access to it.