Yep, pretty much as Rich says.
Our base unit is the Double crown 20x30 ( twice the size of a Crown 15x20 ), so for example out 4 sht is 4 times a Double Crown @ 40x60.
The Quad is 4 times bigger than the Crown hence to give it it's correct title the "Quad Crown". Quads also are available in 60x20 ( it's all about surface area ), and early ones are vertical... And then we have the Quad Royale, @ 40x50.. Clear as mud..