The Middleburg National Horse Show will open Friday and continue through Tuesday at the Foxcroft School, just outside of Middleberg. It will be the first of five major shows in the Washington area.

All five are recognized by the national governing body, the American Horse Shows Association and are top rated with the classification of "A". Two of these events, the Washington International and the World Cup finals, will feature highly acclaimed international riders.

Among the competitors at the Middleburg show will be Rodney Jenkins of Orange, Va., America's leading professional rider.Jenkins has entered horses in both classes, the hunters and jumpers.

In the hunter divisions the horses are judged on their jumping form and smoothness as they negotiate eight fences. The fences are painted in shades of browns and are never more than four feet high in the most advanced division.

The hunter category has several divisions: junior, amateur and green, based on the experience of the horse and rider; and working (performance and appearance of the horse).

But form is of no importance in the jumper classes, where the only thing that matters is that the horse get from one side of a barrier to the other.

In the feature event Monday, the Jumper Stake, Jenkins is scheduled to ride Coastline, owned by Bunny French, and Fair Cast, owned by Joe Fiori.

There are usually eight to 12 brightly colored jumps, starting at four feet. The scoring is on a demerit basis, with four faults scored for knocking down a jump, three for the first time a horse refuses to jump, six for the second refusal and elimination for the third. If, in classes where touches count, a horse touches the barrier with its front legs one fault is scored. One-half is counted for touching with the hind feet. The horse and rider with the lowest score win.

In some classes of this competition, the time taken to negotiate the jumps is factored into the scoring.

Within the jumper division also there are different levels: junior, amateur, preliminary, intermediate and open.

Equitation competition is held for junior riders only. In this division the rider's form, style and ability to control the horse are judged. The type of jumps used in this competition is at the discretion of the management.

The Foxcroft School horse show grounds can be reached by taking Rte. 50 west to Middleburg, turning right at the blinking traffic light in the township and following the signs.

The other area horse shows are Montgomery Country Fair, Gaithersburg, Aug. 24-25; Warrenton horse Show, Warrenton, Va., Aug. 31-Sept. 3; Washington International, Capital Centre, Oct. 21-28; World Cup Finals, Baltimore Civic Center, April 23-27.