To know what it is you're buying, you need to know the different parts of the bridle - the browband, the cheekpieces, the throatlash and the noseband.
The browband isn’t adjustable so it’s important that you have the right size – it should be big enough to allow the headpiece to lie comfortably behind the ears. The browband should also lie just below the base of the ears, without cutting into them.
The cheekpieces play an important role – they determine the level of communication between you and your horse. A properly fitted cheekpiece will allow the bit to just wrinkle the corners of his mouth. It’s also worth checking your horse’s bit is the right size – it should stick out about a quarter of an inch at each side of his mouth. Cheekpieces that are too loose will position the bit too low in his mouth, causing the bit to hit his front teeth and put uncomfortable pressure on his tongue. Cheekpieces that are tight place the bit too high in his mouth, digging into the cheeks, causing pinching.
When it’s done up you should be able to fit the width of four fingers between the throatlash and your horse’s jawbone. If the throatlash is too tight it will put pressure on his windpipe when he flexes to the bit.
A cavesson noseband should lie about two fingers’ width below your horse’s cheekbone – the hard bone that juts out on the side of his face. When fastened you should be able to insert two fingers between the noseband and your horse. Cavesson nosebands are really there for cosmetic rather than functional purposes. Flash nosebands are also popular – the flash strap fastens below the bit and prevents your horse from opening his mouth and evading the bit. The flash strap shouldn’t be fitted too tightly and should only come into play if your horse opens his mouth.