Several kinds of wheat are grown today, each with its own characteristics and uses (see box). Most are species of bread wheat, Triticum aestivum. Their most important distinguishing characteristic is the content and quality of gluten proteins, with high protein content and strong gluten often coinciding with a hard, glassy, translucent grain interior. Hard wheat grains constitute about 75% of the American crop. Soft wheats, which make up 20% of the crop, have a lower amount of somewhat weaker gluten proteins. Club wheat is a distinct species, T. compactum, whose proteins form an especially weak gluten. Durum wheat is another distinct species (T. turgidum durum) used mainly to make pasta.