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Fitting a saddle is always a mystery however, I do find that during the 
winter their coat is long enough to not show the white hairs.  When your 
winter coats sheds off, all of a sudden you have white hairs (a little late, 
I know)!  You mentioned an even sweat pattern.  Make certain that with each 
new saddle you purchase, you ride your horse long enough in the beginning a 
few times to make him sweat under that saddle.  If that doesn't work, borrow 
a pad that you know will make him sweat and ride him with that.  Then check 
you sweat patterns.  If he has an even sweat pattern under the entire portion 
of where the saddle is placed, you should be okay (let me emphasize 
"should").  Still remember that when trying a new saddle, do this during the 
summer so his signs are more reliable and are shown faster.  I have a horse 
that had two dry spots on both sides of his withers.  He was never sore 
there, nor was any white hairs showing so I kept using the saddle hoping for 
the best.  Wrong move.  Come summer when that coat started to shed, their 
they were!  And I am killing myself for it because I should have known!  Just 
my advice, you may receive better!
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