Since 1975, the Kentucky Derby, the first jewel in the Triple Crown, has had a limited field of 20 horses.
Beginning in 2013, the Derby instituted a qualifying system that awards points for top four finishes in 36 prep races.
The first 20 races are grouped into the Prep Series, where the horses pick up 10 points for a win, four for second place, two for third and one point for fourth (10-4-2-1). The only exception is the Breeders' Cup Juvenile, which awards double points (20-8-4-2).
The Prep Series began with the Iroquois at Churchill Downs on September 5 and will wrap up with the Southwest at Oaklawn Park on February 15.
The 16-race Championship Series runs from February 13 with the Risen Star at the Fair Grounds to the Arkansas Derby at Oaklawn Park on April 10. Two of the races have points of 20-8-4-2, seven have 50-25-10-5, and the remaining seven have 100-40-20-10.
The horses with the top 18 point totals will be invited to run in the Kentucky Derby.
Other roads to the Kentucky Derby
There is also a four-race Japan Road to the Kentucky Derby and a seven-race Europe Road to the Kentucky Derby.
The points for those races range from 10-4-2-1 to 40-16-8-4. An invitation to the Derby is extended to the points leader in each of those series.
Given that horses tend to run about one race a month, if that, a victory in one of the 100-40-20-10 races virtually ensures entry into the Kentucky Derby. This means interest is extremely high and wagering is quite active for those races.
The highest point races are 1 1/8 miles in length, a furlong short of the 1 1/4-mile Kentucky Derby. The only exception is the 1 3/16-mile UAE Derby that is run at Meydan Racecourse in Dubai.
The 50-25-10-5 races are mostly 1 1/16 miles, except for the last one, the Sunland Derby on March 30.
The difference in length of the races can be significant as trainers lay out the racing schedules of their horses. While some would rather run at a distance closer to the length of the Derby, others might prefer the shorter race if it better suits their horse's capability or performance history.
Five tracks host Prep Series and Championship Series races, which affords some flexibility to trainers who would prefer to have their horse remain in one location. It also provides them a last chance to qualify if they failed to gain enough points in their first outing at that track.