What is the usual sequence of coal ranks from lignite to anthracite?

The process of coalification describes the transformation of organic material into coal over time, driven primarily by increasing temperature and depth of burial. The usual sequence of coal ranks, ordered from the lowest to the highest degree of coalification, is as follows:12

• Lignite: A very soft, low-rank coal.2
• Subbituminous coal: A rank higher than lignite.12
• Bituminous coal: A mid-to-high rank coal often used for industrial purposes.23
• Anthracite: The highest rank of coal, typically formed under higher temperatures and intense geological deformation.2

As coal progresses through these ranks, its fixed carbon content increases, while its moisture and volatile material content decrease. Generally, the calorific (heat) value of the coal also increases from lignite through bituminous coal.2