20 meter circles
The first, and arguably the easiest, circle to ride in dressage tests is
the 20m circle. This movement is required through Second Level in U.S.
dressage tests and is performed at the trot and canter.
Circles are supposed to be round, which means the horse turns the same
amount at each stride. This is harder than it sounds, especially if your
horse is plagued by "rail magnets": once he's on the rail he doesn't want
to come off! There should be no straight lines or corners involved in riding
circles. At those points of the circle which are tangent to the rail, the
horse should take only one or two steps on the rail before leaving it to
follow the track of the circle.
Look at the diagram above. You can see that in the large arena, which is
20 meters wide and 60 meters long (check your arena
diagrams if you're unsure of the arena dimensions), there is room for
3 separate 20m circles.
When riding a 20 meter circle which begins and ends at C or A, you should
leave the rail as you pass the letter. (Some people say you're "at" the
letter when your horse's shoulder is next to the letter; others say you're
at the letter when your body passes it. It's a pretty fine point 
probably either will work. Just be consistent.)
As you continue on the circle, you'll touch the rail at a point 10 meters from the end of the arena (hurray for circle points!). If your circle begins at B or E, you touch the rail for a
moment at each of these letters before continuing on the circle.
Remember that the corner letters (M, F, H, K) are only 6 meters from the
end of the ring. When riding 20 meter circles at A or C, you need to touch
the rail 4 meters beyond those letters. Use those circle points!
It's a good idea, before it's time to enter the arena for competition, to
find some sort of landmark to help you determine where the 10 meter point
is. Look for an arena post or pylon, a clump of grass, a rock or a pile
of manure  anything that will help you judge where to touch the rail on
each side of your circle.
For circles at A or C, you cross the centerline 20 meters from A or C
(seems obvious, doesn't it?). If you know that the corner letters are 6
meters from the end of the ring, and that the next set of letters is
another 12 meters down the long side, you can find an approximate
crossing place for your circle. Find the SR line for a circle at C (the
VL line for a circle at A) and aim to cross the centerline 2 meters beyond that line. Circle points for the win!
You'll use those same points to find where to cross the centerline for a
20 meter circle at E or B. You'll want to cross the centerline 10 meters
on either side of X: the radius of your 20 meter circle is 10 meters.
Remember that I and L are 12 meters from X. Aim for the centerline 2
meters to the inside of those letters for a great 20 meter circle.
Just to keep things interesting, occasionally a test will ask you to ride a 20 meter circle at R or S or V or P. Note that these are the same circles you would ride if you're asked to ride a 20 meter circle at X. See the diagram below for help.
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Riding a 20 meter circle in the small arena is no different than riding
one in the large arena  the circle size is the same, only the landmarks
change.
Circles at A and C (or X): In the small arena, just like the large, the
corner letters (H, K, M, F) are six meters from the end of the arena.
Therefore, your 20 meter circles at A and C touch the long sides of the
arena 4 meters beyond the corner letters. However, in the small arena you
cross the centerline exactly at X.
Usually it's pretty easy to find X in the small arena at competitions.
Remember, everyone halts there  look for the spot with the most
hoofprints (and manure piles!).
Circles at B and E: When riding a circle which begins at B or E,
your horse's shoulders should leave the track as they pass the letter.
Remember to counteract those "rail magnets" by warning your horse ahead
of time that you're planning to turn.
You'll cross the centerline 10 meters on either side of X. You can eyeball
a spot halfway between X and A, and halfway between X and C, to find that
spot. Or, look for that magic 4metersbeyondthecornerletters spot, and
cross the centerline opposite that. (The corner letters are 6 meters from
the end of the ring. Add another 4 meters to find the ten meter mark. From
that mark, it's another 10 meters to E or B.)
Again, it's a good idea to look for landmarks before you enter the
competition arena.
* * *
From the web: some supercool outside resources (not part of ShortenYourReins.com  and not my fault if they don't work!) that you are sure to find helpful.
How to ride the 20 meter circle.
From 40 Fundamentals of English Riding, from Riding Right Video
