Coal the heat and pressure of deep burial causes the loss of water, methane and carbon dioxide and an increase in the proportion of carbon. Thus first lignite (also called "brown coal"), then sub-bituminous coal, bituminous coal, and lastly anthracite (also called "hard coal" or "black coal") may be formed. The wide, shallow seas of the Carboniferous Period provided ideal conditions for coal formation, although coal is known from most geological periods. The exception is the coal gap in the Permian–Triassic extinction event, where coal is rare. Coal is known from Precambrian strata, which predate land plants—this coal is presumed to
Chinese given name in Mao Tse-tung was the fourteenth generation of such a cycle, but he chose to ignore his family's generational poem to name his own sons. A similar practice was observed regarding the stage names of Chinese opera performers: all the students entering a training academy in the same year would adopt the same first character in their new "given name". For example, as part of the class entering the National Drama School in 1933, Li Yuru adopted a name with the central character "jade" (). There are also other conventions. It is frequently the case that children are given names