Letters were not used until international competitions began with the Olympics in the

1900's. The first competition in 1912 did not have letters.

No Olympics were held in 1916 because of World War I. The letters just "appeared' in

1920 Olympics and no one has ever been able to find the origin although various theories

have been advanced over the years, including one theory that the letters were the first

letter of the names of cities conquered by the Romans.

A second possibility:

"It appears that in the Old Imperial German Court the walls of the Royal Mews/Stable

yard were initially marked with letters indicating where each Courtier and/or riders horse

was to be stood to await its rider.

K = Kaiser/King

F = Furst/Prince

P = Pferdknecht/Ostler

V = Vassal

E = Edeling/Ehrengast/Guest of Honour

B = Bannertrager/Standard Bearer

S = Schzkanzler/Chancellor of Exchequer

R = Ritter/Knight

M = Meier/Steward

H = Hofsmarshall/Lord Chancellor"

However, in the 1920 Olympics, the letters on the centre line were also used: A, D, L, X,

I, G, and C.

There are two sizes of dressage arena - the standard and the long  - and the distance between the markers for each arena is as follows

Standard arena 20m x 40m:Letters are set 6 meters (19.8 feet) in from each corner, and then the rest are 14 meters (39.4 feet) from each other. "A" and "C" are set dead centre 10 meters (33 feet).