M
M is on the second long side just after C, 6 m from the corner at the far short side.
In the arena
Small arena 20 x 40 m
Large arena 20 x 60 m
Good to know
M is 6 m from the corner, directly opposite H, and in line with the centre-line letter G.
Sources: United States Dressage Federation (USDF) · Equine World UK · Premier Equestrian.