David May at The Cumbrian School of Saddlery produces a pattern book http://www.saddleryc...other/tools.htm;. I use Bridlework by Robert Steinke which is a really useful book as it has measurments for sizes from shetland to extra large, and patterns for headcollars, bridles, breastplates, martingales ect; Both of the above are english style, but you could work out western style from the sizing. These 2 books will give you a good basic idea and then you can adapt for each horse, you do find some breeds are different, I make mainly for arabians and they need a large brow band, short cheek pieces, large throatlash, and small nose, as you can see in the photo