The American Orchid Society groups orchid genera into 16 distinct alliances based on a number of factors. (https://www.aos.org/explore-orchids) The alliances are Angraecum (comet orchids), Bulbophyllum, Catasetum, Cattleya, Coelogyne, Cymbidium, Dendrobium, Maxillaria, Oncidium, Phalaenopsis (moth orchids), Pleurothallid, Slipper, Stanhopea, Vanda, Zygopetalum, and Miscellaneous.
Angraecum Alliance
(pronounced: an-GRAY-cum)
A large group of monopodial genera distributed, with the exceptions of Campylocentrum and Dendrophylax, throughout Africa, Madagascar, the islands of the Indian Ocean, and, with one outlier, Sri Lanka. Campylocentrum is found in Central America southward into tropical South America and Dendrophylax is found from Central America across the islands of the Caribbean and into southern Florida with a single species — Florida’s famous Ghost Orchid, Dendrophylax lindenii. Angraecum, Aerangis, Aeranthes, and Jumellea are most commonly encountered in cultivation and Darwin’s orchid, Angraecum sesquipedale, is perhaps the quintessential angraecoid.
Bulbophyllum Alliance
(pronounced: bulb-oh-FILL-um)
The Bulbophyllum Alliance is another very large assemblage of species at one time described in a number of different genera including Cirrhopetalum, Trias, and dozens of other names that have now all been synonymized with Bulbophyllum. It is still common to find plants in the horticultural trade labeled with these older names and will likely be for some time to come. Today, the Alliance consists of one megagenus (Bulbophyllum), containing about 2,200 species distributed from continental tropical Africa, Comoros, Madagascar, Seychelles, Reunion, and Mauritius (about 200 species), India, Nepal, Bhutan, China, Taiwan, Thailand, Malaysia, Indonesia, the Philippines, New Guinea, Australia, New Zealand, and the tropical Pacific islands as far east as Tahiti (about 1,700 species) to the Neotropics (about 100 species). The main centers of diversity are Madagascar (200 species) and New Guinea (600 species). Some of the more common former genera now synonymous with Bulbophyllum include: Cirrhopetalum and Trias.
Catasetum Alliance
(pronounced: cat-a-SEE-tum)
The Catasetum Alliance currently consists of seven genera, Catasetum, Clowesia, Cycnoches, Dressleria, Galeandra, Grobya, and Mormodes. Catasetum and Cycnoches share an apparently unique feature in the orchid family in that these plants produce unisexual flowers. In many species, the male and female flowers may be so remarkably different in color and appearance to have been given two different taxonomic names by early taxonomists. Mormodes produce perfect flowers (having both male and female reproductive structures) however the respective parts mature at different times preventing self-fertilization. The remaining genera produce perfect flowers. All are pollinated by Euglossine bees. All but Dressleria and Grobya are common in today’s orchid collections as are their intergeneric hybrids.
Cattleya Alliance
(pronounced: CAT-lee-ah)
The Cattleya Alliance is a large assemblage of very popular genera and hundreds of intergeneric combinations (nothogenera) distributed throughout the tropical and subtropical areas of the Americas and the Caribbean. The northernmost limit is represented by Epidendrum magnoliae (syn Epidendrum conopseum) extending along the eastern seaboard of United States as far north as coastal North Carolina. In South America, some Cattleya and Epidendrum species are found as far south as Uruguay and northern Argentina.
Coelogyne Alliance
(pronounced: see-LODGE-eh-nee)
The Coelogyne Alliance consists of about 21 genera and approximately 680 species widely distributed in tropical Asia and the Pacific from Sri Lanka, through India, Southeast Asia, the Malay Archipelago, Taiwan, Japan, and the tropical Pacific islands east to Samoa. The genus Coelogyne is the largest with about 500 species and the most widely encountered in cultivation (now including the former genera Bracisepalum, Bulleyia, Chelonistele, Dendrochilum, Dickasonia, Entomophobia, Geesinkorchis, Gynoglottis, Ischnogyne, Nabaluia, Neogyna, Otochilus, Panisea, and Pholidota).
Cymbidium Alliance
(pronounced: sim-BID-ee-um)
The Cymbidium Alliance is composed of 11 genera, nine distributed from the northwestern Himalayas to Japan and south through Indochina and Malesia to the Philippines, New Guinea, and Australia. Ansellia is endemic to Africa south of the Sahara and Imerinaea is endemic to Madagascar.
Dendrobium Alliance
(pronounced: den-DRO-bee-um)
The Dendrobium Alliance at one time consisted of a number of different genera; however, as our understanding of genetic relationships between plants has evolved, these former genera have been reduced to synonymy with Dendrobium. Although, it is common to find plants in the horticultural trade labeled with the old names and likely will be for some time to come. Today, the Alliance consists of one megagenus (Dendrobium) comprising some 1,450 species, subdivided into 30 sections. Some of the more common former genera now synonymous with Dendrobium include: Cadetia, Dendrocoryne, Diplocaulobium, Cadetia, and Flickingeria. The genus ranges from Sri Lanka throughout tropical Asia and the Pacific region, north to Japan, east to Tahiti, and south to New Zealand.
Maxillaria Alliance
(pronounced: max-ih-LARE-ee-uh)
The Maxillaria Alliance consists of 12 genera with about 720 species distributed throughout the Neotropics. There are two centers of distribution, one in Andean South American and the other in the Brazilian Atlantic rainforest. The five most commonly grown genera in the Alliance are listed first below. It should be noted that the Maxillaria genus currently includes a number of once-recognized genera now considered synonyms – Brailiorchis, Camaridium, Christensonella, Cryptocentrum Heterotaxis, Hylaeorchis, Inti, Mapinguari, Maxillariella, Mormolyca, Nitidobulbon, Ornithidium, Pityphyllum, Rentinantha, Sauvetrea, and Trigonidium).
Oncidium Alliance
(pronounced: on-SID-dee-um)
The Oncidium Alliance is a large group of about 69 genera and 1,600 species stretching from the southern tip of Florida and northern Mexico throughout the Caribbean and Central America to southern Peru, Bolivia, and northern Argentina.
DNA analyses have led to the lumping of a number of former genera into Oncidium, making it now the largest genus in the Alliance with about 520 species. The genera now considered to belong to Oncidium are Odontoglossum, Sigmatostalix, Cochlioda, Symphyglossum, Mexicoa, Miltonioides, Solenidiopsis, Chamaeleorchids, Collare-stuartense and Heteranthocidium.
Phalaenopsis Alliance
(pronounced: fayl-eh-NOP-siss)
The Phalaenopsis Alliance at one time consisted of a number of genera including Doritis, Kingidium, Kingiella, and several other genera. As the understanding of genetic relationships between plants has evolved, the taxonomy of these former genera has been reduced to synonymy with Phalaenopsis although it is common to find plants in the horticultural trade labeled as Doritis and Doritaenopsis and likely will be for some time to come.
Pleurothallid Alliance
The Pleurothallid Alliance comprises about 38 genera of generally miniature orchids with around 4,000 species distributed from southern Mexico and southern Florida throughout Central America, and the Caribbean southward throughout much of South America. The center of diversity is in the Andean Mountains. Pleurothallis is the largest genus in the Alliance although Dracula, Dryadella, Lepanthes, Masdevallia, Restrepia, and Stelis are the most popular with horticulturalists.
Slipper Alliance
The Slipper Alliance is distributed throughout the temperate regions of Eurasia and North America, Central America, south to Brazil and Bolivia, tropical Asia from India to Taiwan, southeast Asia from Indochina across to the Philippines, New Guinea, and the Solomon Islands.
Cypripedium
(pronounced: sip-ree-PEE-dee-um)
From the Greek Kypris, meaning Venus, and pedion, meaning sandal, alluding to the beautiful slipper shape of the lip. A panboreal genus of 45 species extending southwards into the mountains of China and Mexico. Common names: Lady’s Slipper Orchid, Moccasin Flower.
Mexipedium
(pronounced: meks-ee-PEED-ee-um)
Paphiopedilum
(pronounced: paff-ee-oh-PED-eh-lum)
A genus of 60 species recorded from South India to the Philippines and New Guinea
Phragmipedium
(pronounced: frag-meh-PEE-dee-um)
Selenipedium
(pronounced: sel-en-eh-PEE-dee-um)
From the Greek selene, meaning moon or crescent, and pedilon, meaning sandal, for the corniculate lip. A genus of six species distributed from Panama to Ecuador, Brazil, and Trinidad.
Stanhopea Alliance
(pronounced: stan-HOPE-ea)
The Stanhopea Alliance consists of some 20 genera and about 270 species distributed throughout the humid tropics from the Tropic of Cancer in Mexico to the Tropic of Capricorn in northern Argentina and southern Brazil.
Pollinated by Euglossine bees, members of the Stanhopea Alliance possess powerful, resinous fragrances.
Vanda Alliance
(pronounced: VAN-duh)
This group of monopodial orchids comprises about 80 genera with about 1,550 species, distributed from India and Sri Lanka throughout tropical Asia and the Pacific region, north to Japan, east to Tahiti, south to New Zealand, with two species (Acampe and Taeniophyllum) in tropical and southeastern Africa, and Acampe also found in Madagascar. Pollinated by Euglossine bees, members of the Stanhopea Alliance possess powerful, resinous fragrances.
Neofinetia falcata (Vanda falcata)
https://www.aos.org/neofinetia-falcata
Sarcochilus
(pronounced: sar-koh-KYE-luss)
This is a small genus of miniature monopodial orchids, mostly from Australia.
Zygopetalum Alliance
(pronounced: zy-go-PET-a-lum)
The Zygopetalum Alliance comprises about 35 genera distributed throughout the American tropics from southern Mexico and the West Indies to Brazil, Paraguay, Bolivia, and northern Argentina. The northernmost species belong to the genera Kefersteinia and Stenotyla. Warrea warreana and Zygopetalum maxillare extend to Paraguay and Argentina.
Miscellaneous Alliance
This group of approximately 160 genera consists of those that do not fit into other horticultural alliances as well as the terrestrial genera including all the European terrestrials, Australia terrestrials and the tropical and subtropical jewel orchids.