DeltAlum has it mostly right, but 2 half-yards is more beer than 1 yard.
Both a yard glass and a half-yard glass have a large bulb at the bottom, a flared top, and a tube about 2 inches in diameter connecting them. Each sits in a wooden stand - a yard stand has to rest on the floor, while a half-yard can rest on a table. A yard is 3 feet long from top to bottom, a half-yard is 1.5 feet long. (hence the name) A yard holds about 60 oz worth of beer, a half-yard holds maybe 36 oz.
If you want to see something impressive, get a yard of Guinness. I've seen summer storms that don't look that bad.
