Rye flour is made from the rye grain. It is similar to wheat flour, but with a lower gluten content and a denser texture. Rye flour can be used to make rye bread, which is dark and dense. This is common in Germany, where they call this type of bread pumpernickel. Rye can also be mixed with wheat, or used to make crackers. There is more information about rye bread on wikipedia (opens in new window).