18.1. The name of a family is a plural adjective used as a substantive; it is formed from the genitive singular of a legitimate name of an included genus by replacing the genitive singular inflection (Latin -ae, -i, -us, -is; transliterated Greek -ou, -os, -es, -as, or -ous, including the latter's equivalent -eos) with the termination -aceae. For generic names of non-classical origin, when analogy with classical names is insufficient to determine the genitive singular, -aceae is added to the full word. For generic names with alternative genitives the one implicitly used by the original author must be maintained.
Ex. 1. Family names based on a generic name of classical origin: Rosaceae (from Rosa, Rosae), Salicaceae (from Salix, Salicis), Plumbaginaceae (from Plumbago, Plumbaginis), Rhodophyllaceae (from Rhodophyllus, Rhodophylli), Rhodophyllidaceae (from Rhodophyllis, Rhodophyllidos), Sclerodermataceae (from Scleroderma, Sclerodermatos), Aextoxicaceae (from Aextoxicon, Aextoxicou), Potamogetonaceae (from Potamogeton, Potamogetonos).
Ex. 2. Family names based on a generic name of non-classical origin: Nelumbonaceae (from Nelumbo, Nelumbonis, declined by analogy with umbo, umbonis), Ginkgoaceae (from Ginkgo, indeclinable).
18.2. Names intended as names of families, but published with their rank denoted by one of the terms "order" (ordo) or "natural order" (ordo naturalis) instead of "family", are treated as having been published as names of families (see also Art. 19.2).
Ex. 3. Cyperaceae Juss. (1789) and Xylomataceae Fr. (1820) were published as "ordo Cyperoideae" and "ordo Xylomaceae".
18.3. A name of a family based on an illegitimate generic name is illegitimate unless conserved. Contrary to Art. 32.1(b) such a name is validly published if it complies with the other requirements for valid publication.
Ex. 4. Caryophyllaceae Juss., nom. cons. (from Caryophyllus Mill. non L.); Winteraceae Lindl., nom. cons. (from Wintera Murray, an illegitimate synonym of Drimys J. R. Forst. & G. Forst.).
18.4. When a name of a family has been published with an improper Latin termination, the termination must be changed to conform with the rule, without change of the author's name or date of publication (see Art. 32.6).
Ex. 5. "Coscinodisceae" (Kützing 1844) is to be accepted as Coscinodiscaceae Kütz. and not attributed to De Toni, who first used the correct spelling (in Notarisia 5: 915. 1890).
Ex. 6. "Atherospermeae" (Brown 1814) is to be accepted as Atherospermataceae R. Br. and not attributed to Airy Shaw (in Willis, Dict. Fl. Pl., ed. 7: 104. 1966), who first used the correct spelling, or to Lindley (Veg. Kingd.: 300. 1846), who used the spelling "Atherospermaceae".
Ex. 7. However, Tricholomées (Roze in Bull. Soc. Bot. France 23: 49. 1876) is not to be accepted as "Tricholomataceae Roze", because it has a French rather than a Latin termination. The name Tricholomataceae was later validated by Pouzar (1983; see App. IIA).
18.5. The following names, of long usage, are treated as validly published: Palmae (Arecaceae; type, Areca L.); Gramineae (Poaceae; type, Poa L.); Cruciferae (Brassicaceae; type, Brassica L.); Leguminosae (Fabaceae; type, Faba Mill. [= Vicia L.]); Guttiferae (Clusiaceae; type, Clusia L.); Umbelliferae (Apiaceae; type, Apium L.); Labiatae (Lamiaceae; type, Lamium L.); Compositae (Asteraceae; type, Aster L.). When the Papilionaceae (Fabaceae; type, Faba Mill.) are regarded as a family distinct from the remainder of the Leguminosae, the name Papilionaceae is conserved against Leguminosae.
18.6. The use, as alternatives, of the names indicated
in parentheses in Art. 18.5 is authorized.