Youre probably wondering what the "D" stands for in 600D. D stands for denier. Denier is simply a unit of measurement which describes linear mass density in a given material. A low denier count in fabrics tends towards light, smooth, silky, sheer, and likely soft. A high denier count tends to be sturdy, thick, and more durable. So as tempting as it is to purchase a less expensive 210D, 300D or 420D cover, theyre just not as strong and durable as a 600D cover.