Lignite: Increased pressures and heat from overlying strata causes buried peat to dry and harden into lignite. Lignite is a brownish-black coal with generally high moisture and ash content and lower heating value. However, it is an important form of energy for generating electricity. Significant lignite mining operations are located in Texas, North Dakota, Louisiana, and Montana.

Subbituminous Coal: Under still more pressure, some lignite was changed into the next rank of coal subbituminous. This is a dull black coal with a higher heating value than lignite that is used primarily for generating electricity and for space heating. Most subbituminous reserves are located in Montana, Wyoming, Colorado, New Mexico, Washington and Alaska.

Bituminous Coal: Even greater pressure results in the creation of bituminous, or “soft” coal. This is the type most commonly used for electric power generation in the U.S. It has a higher heating value than either lignite or subbituminous, but less than that of anthracite. Bituminous coal is mined chiefly in Appalachia and the Midwest. Also used to make coke.
Anthracite: Sometimes also called “hard coal,” anthracite forms from bituminous coal when great pressures developed in folded rock strata during the creation of mountain ranges. This occurs only in limited geographic areas - primarily the Appalachian region of Pennsylvania. Anthracite has the highest energy content of all coals and is used for space heating and generating electricity.