For at least 70 years most coal geologists and coal petrologists have agreed that there is some sort of a general progression from peat through various coals all the way to anthracite. The terms which have been applied classically to this sequence are peat - lignite - subbituminous - bituminous - semi-bituminous - semi-anthracite - anthracite - and meta- anthracite. At one time some authors felt that specific vegetational types gave rise to different rank coals. Most people do not hold with this interpretation at the present time. Throughout the sequence noted above, many changes take place and it is the measurement of these changes, or the parameters which are used to measure these changes, which give rise to the word rank.
The International Committee for Coal Petrology in the second edition of the International Handbook for Coal Petrology (1963) agreed that the term rank should be accepted as an international scientific term. They suggest "degree of coalification" as a synonym for rank. As a definition for the word they suggest, "describes the stage of carbonification attained by a given coal". The tricky part, of course, now becomes how do we measure the degree of carbonification or coalification.