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In botany, the phrase ordo naturalis, "natural order", was once used for what today is a family. Its origins lie with Carolus Linnaeus who used the word when he referred to natural units of plants. In his famous works the Systema Naturae and the Species Plantarum, arranged according to his artificial "Sexual system", Linnaeus used the word "ordo" for an artificial unit: in these works only genera and species (sometimes varieties) were "real" taxa. It is only in his lesser known works that Linnaeus wrote about natural botanical units, at a rank higher than genus.
In nineteenth century works such as the Prodromus of de Candolle and the Genera Plantarum of Bentham & Hooker, the word ordo did indicate taxa that are now given the rank of family. Contemporary French works used the word "famille" for these same taxa. In the first international Rules of botanical nomenclature of 1906 the word family (familia) was assigned to this rank, while the term "order" (ordo) was reserved for a higher rank, for what in the nineteenth century had often been named a cohors (plural cohortes).
The International Code of Botanical Nomenclature provides for names published in the rank of ordo naturalis in Art 18.2: normally, these are to be accepted as family names.
Some plant families retain the name they were given as pre-Linnaean natural groups, recognised by Linnaeus as "natural orders" in his natural classification (e.g. Palmae or Labiatae). Such names are known as descriptive family names.