American whiskey includes both straights and blends. To be called "straight" the whiskey must be one of the "named types" listed in the federal regulations and aged in oak casks for at least two years. The most common of the "named types" are Bourbon, which must be at least 51% corn (maize); rye, which must be at least 51% rye, and corn, which must be at least 80% corn. All straight whiskeys except straight corn whiskey must be aged in new casks that have been charred on their inside surface. American blended whiskeys combine straight whiskey with un-aged whiskey, grain neutral spirits, flavorings and colorings. These definitions are part of U.S. law. Not defined by the law but important in the marketplace is Tennessee whiskey. It is identical to bourbon in every important respect. There are other, minor, variations to the above, but they are not significant in the marketplace. Grain whisky differs from malt in that it is usually made from corn, maize, or other grains rather than malted barley. It is distilled in continuous distillation process stills known as Coffey stills instead of the pot still used for malt whisky. Pure pot still whiskey refers to Irish whiskey made from a combination of malted and unmalted barley and distilled in a pot still.