K-9 Komfort Sizing is a true-fit sizing. We measure our sizes from the end of the buckle to the middle pre-punched hole. A 21" size will then fit neck sizes from 19" to 23". Customers simply measure their dog's neck and order the size closest to that measurement. We recommend including this information in your sales vectors so that your customers are clear on how sizing works (since their is little to no standardization in collar sizing, this is always a point of contention - making it clear from the onset increases satisfaction and decreases returns regardless of the collar manufacturer).