Class  in thoroughbred horse racing has been defined in many different ways because it is, as Ainslie said, easy to recognize and hard to define. Class truly encompasses many other handicapping factors, such as speed, pace ability, horse sense in the literal meaning of the expression, and more. Perhaps the simplest way to define class is to say that it is a horse's competitive will to win as demonstrated by its racing record and/or style. 

Cretainly, at least part of a horse's class is quantifiable, as seen in the prices assigned to claiming horses. All other things being equal, a $10,000 claimer should easily beat a field of $5,000 claimers, and in turn, be facing the behinds of most of the field when facing $25,000 claimers. 

But as you know, most of the time all things in racing are not equal! 

Andy Beyer's par, or average, running times for the most part clearly demonstrate that classier horses run faster than cheaper ones, and this is certainly a valid way of measuring class among the claimers. James Quinn's fine work, The Handicapper's Condition Book, in both its original form and more recent revision, is one of the best sources for beginners and seasoned pros alike who wish to review the class factor, especially as it applies to the better animals that race in allowances, stakes, handicaps, and Graded races. 

Let's review some common situations that have been around for years, but which must be examined in view of the present state of racing today and put aside the sometimes conflicting claims that spring from studies of various databases. 

CLASS RAISES

In claiming races, I usually assume a trainer is running a horse over his head primarily to race it into condition UNLESS the horse showed a sign of improvement that can be the tipoff to a peak effort coming up. Specifically, I look for uncharacteristic speed early in the race or a noticeable gain in the stretch. I restrict this to older horses, 4 and up. Three year olds are truly developing animals and are often subject to spurts in development that are  tough to detect. 

If you play just one track or circuit, keeping a notebook for the various claiming levels will tell you after a few weeks if there is any real classdifference between adjacent levels of horses. At some bullrings, for example, $2,500 and $3,500 horses are virtually the same group, running one week for one price and the next at the other! 

CLASS DROPS

I generally discount horses that are dropping in class from win consideration unless they are being handled by one of the top trainers at the track who I know (from records) is smart enough to know when he's been placing a horse in to races that are just a bit too tough. Why? Because you don't take a horse that has proven it can win at a certain level and offer him for sale at a much cheaper price unless the horse's ability or health is in question. It really IS that simple. 

"Yeah, George, but what if the trainer is dropping him to try to 'steal' an easy purse?"

Fine. Let him beat me. If the animal really does have talent, another trainer will be training him soon, and the former trainer won't have to chance to do it again, most likely. 

MOVING FROM CLAIMING TO ALLOWANCE (AND VICE-VERSA)

My experience has been that the class of the track often determines how to handicap this type of horse. At the biggest and smallest tracks, I generally look very hard at horses moving from a claiming into an allowance race, if the horse has been a hard knocker with a good record of wins and places. The trainer and/or owner is trying for more money via a bigger purse and not afraid to admit it by making this move. Better yet if (1) the horse loses its first such attempt while obviously trying, and comes back in another soon after, and (2) if the field is large, eliminating the possibility that the horse may be a "filler" to help the racing secretary avoid a 4 or 5-horse race. 

From allowance to claiming: once again, for me, let him beat me--if he can! The move downward indicates a less-than-adequate ability and a willingness to let the horse go in the sales sense of the expression. Prove to me the horse should be running at a cheaper level. There are plenty of simulcast races I can look at and bet while I watch to see what happens with one like this. 

STAKES, HANDICAPS, GRADED RACES

If these races, especially at the major tracks, are truly offering the best of the best, as usually happens from just after the Triple Crown races through the Breeder's Cup by my calendar, the deciding factors are condition and money. Almost all of the horses in these races are classy animals. 

MONEY VALUES AS CLASS INDICATORS

They work. 

And they don't work. 

This is not doubletalk; it's one of the best examples of how trying good handicapping can be. Both average purse value and earnings-per-start can be reliable numerical indicators of class, IF and only if, you can find the races that have either ridiculous amounts added for state-bred wins or races at tracks where the purses are inflated way beyond the rest of the circuit because of slot machine-supplemented purses. 

Small tracks and major tracks have only a few things in common, but it's at these tracks where money values seem to have worked best for me. It's easier to follow the money trail throughout the season and spot the occasional Big Win that can throw off an otherwise valid figure. 

This "launching pad" is a very detailed one, and these situations represent only the most common ones. No other factor in handicapping becomes more murky, since age, sex, condition, and even distance at times are intertwined and almost inseparable from the class factor.