If you are buying a horse, your best bet is to take a knowledgeable person with you. Though you can learn quite a bit on your own, it is difficult for an inexperienced horse buyer to catch all of these areas while being entranced by their prospective new horse.
One of the most important criteria in selecting a horse for purchase is conformation, or its physical appearance. While it could be assumed that most horses with several years’ seasoning and past performance have acceptable conformation, your goal in selection should always be to find the best conformed horse possible, regardless of past performance. The reason? Horses with less-than-perfect conformation may encounter health problems as they mature or when stressed through competition.
Rating conformation depends upon objective evaluation of the following four traits: balance, structural correctness, degree of muscling, and breed and sex characteristics. Of the four, balance is the single most important, and refers to the structural and aesthetic blending of body parts. Balance is influenced almost entirely by skeletal structure.
To gain a better understanding of ideal balance in an American Quarter Horse, there are several helpful ratios which may be drawn in your mind’s eye. Start by viewing a horse from its profile, and imagining a straight line determining length of back (the distance from point of withers to croup) and one along the length of underline (point of elbow to stifle).
Ideally, the length of back should be shorter than that of the underline. Next, draw an imaginary line down the top line of the neck (the distance from poll to withers) and the bottom line (the distance from throat latch to neck/shoulder junction). Ideally, the top-to-bottom-line ratio of neck should be 2-to-1. Horses which deviate greatly from these two important ratios, becoming 1-to-1, are often deemed unbalanced.
Here is a 10 minute video that illustrates all of this with pretty good detail.