A class whose members have some internal morpheme in common, e.g. 'dancer', 'player', 'walker', 'worker'. - Nida (1949), a pag.91
An 'internal distribution class' consists of a set of forms all of which have the same morpheme for one of the immediate constituents. Members of an internal distribution class must consist of at least two morpheme. - Nida (1949), a pag.110