There are 2 measureent systems in current use:
-the “American” system with 2″ size increments (70-72-74, etc) and measured from the center of the chest to the butt crack;
-the “European” system with 3″ increments (69-72-75 etc.) and measured from
the center of the chest to back edge of a back leg.
30 years ago, all the blankets used the 2″ increment system, but then some smart person realized that the 2″ system requires 9 blankets to cover the standard range of horse sizes (68, 70, 72, 74, 76, 78, 80, 82, 84), while the 3″ increment system requires only 6 (69, 72, 75, 78, 81, 84). Many blanket manufacturers have converted to the 3″ system as they can cover the range with fewer blankets, which means fewer sizes to be stocked by the manufacturers and the stores, both store-front and on-line. To add to the confusion, each manufacturer has their own specifications and patterns, and while a 74″ in one brand could fit perfectly, that does not mean that every 74″ will fit.
Just to even further confuse things, blankets that are sized according to measurements from the center of the chest to the butt crack don’t actually go around the back end of the horse, they stop a bit past the back edge of the back leg. So if you measure a that “74” inch blanket from the center front, which is actually the bar the tongue rests on for blankets with standard front buckles and straps, to the back end of the blanket, it will not measure measure 74″. It will measure somewhere between 70 & 72, because you measured to the butt crack, but the blanket does not turn the corner at the rump, as it does for the chest. Many of the blankets that use the 3″ increment system will tend to measure larger than the blankets from the 2″ increment system. The major names for the the 3″ increments are Weather Beeta and Horse Ware of Ireland (Rambo, Rhino, Amigo). Schneiders makes some blankets using the 2″ system, others using the 3″ system. Some blankets are cut to fit Quarter Horses, but don’t fit Arabs and many TB’s well.
There is no easy answer to your question – – one’s best bet is too find a brand and a blanket line that fits well and try to stick to it, as not all blankets by a manufacturer will be cut to fit the same horse shape. I’m not sure I have answered your question, but hopefully I have given you some ideas and things to explore on blanket fit.
Also keep in mind that if the fit is even slightly “off”, it can be uncomfortable for the horse, with some horses being very tolerant of fit and other with zero tolerance. Lacking fingers, horses will often try to move the blanket into a more comfortable position using their teeth, with predictably dire results. Also pay attention to the D-rating of T/O material, as it is a measure of the sturdiness of the material. Anything less than 1200-D does not hold up under daily wear for most horses, and some horses do better with 1680-D or 2000-D.