A horseplayer studying the past performances for the Lady Baltimore Handicap at Pimlico last summer might have had trouble choosing between Pretty Does and Lady of Promise. The fillies had raced against one another two weeks earlier, and Pretty Does had won by a nose.

But anyone who had seen that prior race would have known that the rematch was actually a mismatch. Lady of Promise had an extremely tough trip when Pretty Does beat her. Breaking from the outside post position at 1 1/16 miles, she was parked four horses wide at the first turn, contended for the lead along the backstretch, then accelerated when she was three-wide on the final turn to take the lead.

Meanwhile, Pretty Does had been held far behind this battle for the lead. She advanced along the rail on the backstretch and the turn, had clear sailing most of the way and caught Lady of Promise at the wire.

A trip handicapper making notes on his program might have written "rail" for Pretty Does and "4FT, 3T" for Lady of Promise, indicating her position on both turns. As he consulted these notes before the Lady Baltimore Handicap, he would know that Lady of Promise had had the much tougher trip and had run a far superior race. Their rematch figured to end much differently, and it did. Since no horse with early speed was there to press her, Lady of Promise took the lead easily and coasted to a six-length victory. Pretty Does broke from an outside post, lost ground most of the way and finished 19 lengths behind the winner.

These results suggest just how dramatically the routine development of a race can affect its outcome. Neither Pretty Does nor Lady of Promise encountered any extraordinary trouble in either race, but different trips produced a 19-length difference in their relative performances.

The principal component of a horse's trip--the ground he loses on the turns--is an important factor that many lifelong thoroughbred fans have never appreciated. But bettors who learned the game at harness tracks know that a thoroughbred who circles the field five horses wide would lose enough ground to make the difference between victory and defeat in most cases. They were among the original trip handicappers.

Some horseplayers try to calculate the effect of ground loss with mathematical precision; the New York handicapper Len Ragozin computes speed figures that are adjusted for ground loss. But it isn't really possible to treat this factor with exactitude, because the cost of racing wide differs at various tracks. At Belmont Park, horses can race very wide around the sweeping turns without losing momentum. But at tracks such as Pimlico, Charles Town and Timonium, which have especially sharp turns, horses rarely win by circling the field. At these places, handicappers must know where the horses were on the track in their prior starts if they hope to bet intelligently.

On any turf course, trips and ground loss are crucial. The majority of good trip handicappers prefer races on grass and have their greatest successes with those races, because the crowded fields and tight turns make trips all-important. Horses that run on grass are often rematched from one week to the next. If two horses finished a length apart in their previous meeting, but one of them hugged the rail and the other raced in the 3-path all the way, the handicapper who possesses this information has a big edge.

Besides ground loss, two other factors may influence a horse's trip significantly: pace and track bias. Both should be very familiar to most horseplayers. If the inside part of a track is harder and faster than the outside--or conversely, if it is so deep that horses bog down along the rail--a handicapper must view all performances in the light of the bias.

The importance of pace has been recognized and understood by most horseplayers for centuries (although this horseplayer was a late convert). It should be fairly self-evident that an animal subjected to pressure at every stage of a race has a tougher trip than a front-runner who can take a comfortable early lead or a stretch-runner who makes a late move after the leaders have run each other into defeat.

There is little about trip handicapping that is exotic or unfamiliar. What is difficult for most horseplayers is to accept its conclusions, to believe that even though Horse A beat Horse B by 10 lengths, B is the superior animal and should be bet accordingly. For most of my handicapping life, I had been a literal-minded student of speed figures; if one horse ran faster than another, I believed in his superiority. An expert trip handicapper tried to persuade me by saying, somewhat cryptically, "I don't care how fast they run; what matters is how they run fast."

But I remained unconvinced until the winter of 1980. Every one of the important February prep races for 3-year-olds that led up to the Flamingo Stakes was strongly influenced by trips. And the Flamingo itself was like a final examination for a fledgling student of trip handicapping.

Feb. 15: Against a weak field on a speed-favoring track, Colonel Moran dusts off one weak challenger, then skims along the rail to win by 10 lengths and miss the track record by only one-fifth of a second. The performance is eye-catching, but with such an easy trip, the horse has proved little about his capabilities.

Feb. 19: Greentree Stable's highly regarded colt, Prince Valiant, wins an allowance race by 4 1/2 lengths over Koluctoo Bay. The 1 1/6-mile distance starts at the turn and favors horses with inside posts; moreover, the inside part of the track seems generally advantageous. Prince Valiant hugs the rail, while Koluctoo Bay's jockey seems content to stay three-wide all the way.

Feb. 20: Superbity and Irish Tower engage in a suicidal duel for the lead in the Everglades Stakes, battling through a half mile in 45 1/5 seconds, while Rockhill Native sits third, five lengths behind in perfect position. The 2-year-old champion of the previous season passes the tired leaders on the turn and scores a 1 1/2 length victory after the quintessential perfect trip. Superbity and Irish Tower hang on well after their exertions, and look as impressive in defeat as Rockhill Native does in victory.

Two weeks later, the crowd at the Flamingo Stakes bet the contenders in a predictable fashion, on the basis of the way they had finished their previous races, not on the basis of how well they had run. Rockhill Native, beneficiary of the best trip any horse could ever want, was the 7-5 favorite. Colonel Moran, who had coasted along the speed-favoring rail in his last start, was 3-2. Prince Valiant was the third choice. Superbity and Koluctoo Bay, the two horses in the field who had run creditably despite difficult trips, were ignored.

Superbity ran Colonel Moran into defeat and drew away to an easy six-length victory, while Koluctoo Bay rallied to finish second. When this perfectly logical exacta combination paid $265.80, another convert was won to the art of trip handicapping.