This overview of the APG classification system (1998. Annals of the Missouri Botanical Garden 88: 531-553) is derived from a copy of the published document made available by the Royal Botanical Gardens at Kew. Another summary of this classification system is available from the Department of Systematic Botany, Uppsala University.
Based on a cladistic methodology, the groupings present in this system are viewed by the authors as monophyletic 'clades'. The higher levels are described as 'informal' and many families and orders are unassigned to higher level groupings. Thus, comparison between this system and functional (complete) classification systems produced by Cronquist, Thorne, and Takhajan - an objective of the Flowering Plant Gateway - is difficult and, to some extent, forced. However, the APG system is now included as a comparative element of this system, with informal higher level groups roughly placed in concordance with those employed by the other systems.
The family listing pages for elements of the three angiosperm groups recognized by the APG system feature pointers to pages of the other systems (Cr - Cronquist, Th - Thorne, Ta - Takhtajan) that list a given family or, in cases where the APG family is treated as a synonym, the accepted family name for a given system preceded by '='. An '+' preceding an APG listed family denotes those "small families that may be reduced to synonymy of their sister group the the latter consists of a single family". The listing indents these below their sister family. The program that generates the eight family listing pages, apgwww1.ipf, includes new code that allows use of higher level taxonomic names used by a given system in the pointer to that system. This feature, and references to the APG system will be added to programs that produce 'group' pages for the other systems when time permits.