Corners and half-circles

Common practice dictates that, when you're riding dressage tests (and schooling at home, because please oh please don't wait to practice corners until you get to the showring!) your corners don't need to be any larger than the smallest sized circle you must perform in the test. Under this practice, under current USEF/USDF Intro and Training Level tests, you wouldn't ever have to ride a corner -- you could just coast around the ends of the ring in 20 meter half circles.

That idea falls flat the moment you have to turn from the centerline onto a long side (which, remember, is only 10 meters from the centerline). From the beginning of your test-riding career, you'll have to be able to ride at least 10 meter half-circles to navigate around the dressage arena.