This is an extract I found on google : Exercise sheets (also called quarter sheets) are a shortened rug, sold in smaller sizes than rug sizes - Weve put together this handy size guide to help. To check the correct size of exercise sheet to fit your horse, measure from the highest point of your horse's withers to the root of the tail with a soft tape measure. The table gives a rough guide on which size should best fit your horse with a comparison to rug size, making it much easier to choose the correct size - see attached picture