A general guide to ensuring your horse’s bit is at the correct height in the mouth would be to aim for ½ – 1 lip wrinkles at the corners of the mouth. Please bear in mind that mouth conformation varies. Horses with fat and fleshy lips may appear to have many wrinkles at the corner or the mouth without much cheek tension, whereas a thin lipped horse may only show one wrinkle under a large amount of cheek piece tension.
Some horses are also much shorter from the corner of the lip to the muzzle (‘short smile’). In this case it is often tempting to shorten the cheek pieces excessively. However, this can create unnecessary pressure in the commissures of the lip, leading to discomfort, may predispose to rubs and masking of the rein aids.
When fitting a bit with a fixed cheek, such as the Eggbutt Snaffle Bit, the lip should gently brush up against the butt end ensuring a snug, although not tight, fit.
If using a loose ring, the general rule would be to have clearance either side of no more than one eighth of an inch from the corner of the lip up to the hole that the ring passes through.
The fit of the bit needs to be assessed at rest and then with a contact as some designs will shorten up more than others under rein contact. It is quite common for horse riders to employ loose rings that are too big for the mouth. This causes the mouthpiece to slide across the mouth as the rider changes bend or direction. The resulting friction often causes rubs in the commissures of the mouth. The lozenge can also cause damage under these circumstances.