Classifications of fairies
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The fairies of Irish, English, Scottish and Welsh folklore have been classified in a variety of ways. Two of the most prominent categories, derived from Scottish folklore, are the division into the Seelie Court and the Unseelie Court.
These categories may reflect an earlier (medieval) classification of the Germanic elves of light and darkness (Ljósálfar and Dökkálfar), as the corresponding Insular Celtic "fair folk" do not appear to have a comparable division. William Butler Yeats, in Irish Fairy and Folk Tales, further divided them into the Trooping Fairies (appearing in splendid processions known as fairy rades) and the Solitary Fairies (mischievous spirits appearing on their own). Katharine Mary Briggs noted that a third distinction might be needed for "domesticated fairies" who live in human households (see household spirit).
Seelie and Unseelie
The Northern and Middle English word seely (also seily, seelie, sealy, and "seely"), and the Scots form seilie, meaning "happy", "lucky" or "blessed" and unseely meaning "unhappy", "misfortunate" or "unholy" are derived from the Old English sǣl and gesǣlig. The Modern Standard English word silly is also derived from this root and the term "seely" is recorded in numerous works of Middle English literature such as those by Geoffrey Chaucer. Many ballads and tales tell of "Seilie wichts",  a Lowland Scots term for fairies. In Wales there were said to be two fairies or elves called Silly Frit and Sili go Dwt whose names represent a borrowing of the adjective silly (in this case meaning happy) as applied to fantastical beings from its usage on the English marches bordering Wales rather than the Anglo-Scottish border. The former name is purely English while the latter is a corruption of English fairy names featuring "tot" (such as Tom Tit Tot) as an element.
Seelie and Unseelie Courts
In Scottish folklore, fairies are divided into the Seelie Court (more beneficently inclined, but still dangerous), and the Unseelie Court (more malicious).
The Seelie Court were described as those fairies who would seek help from humans, warn those who have accidentally offended them, and return human kindness with favors of their own. Still, a fairy belonging to this court would avenge insults and could be prone to mischief. Seelies were known for playing pranks on humans and having a light hearted attitude, forgetting their sorrows quickly and not realizing how they affect the humans they play pranks on. Other names for the Seelie Court included The Shining Throne, The Golden Ones, and The Summer Court.
The Unseelie Court, conversely, was used to describe the darkly-inclined fairies. Unlike the Seelie Court, no offense was deemed necessary to bring down their assaults. As a group (or "host"), they were thought to appear at night and assault travelers, often carrying them through the air, beating them, and forcing them to commit such acts as shooting at cattle. In Scotland they were seen as closely allied with witches. Like the beings of the Seelie Court who are not always seen as benevolent, neither were the fairies of the Unseelie Court always malevolent. Folklore includes stories of Unseelies becoming fond of particular humans if they are viewed as respectful, and choosing to make them something of a pet. Some of the most common characters in the Unseelie Court were bogies, bogles, boggarts, abbey lubbers and buttery spirits.[Note 1]
In other cultures
In the French fairy tales as told by the précieuses, fairies are likewise divided into good and evil, but the effect is clearly literary. Many of these literary fairies seem preoccupied with the character of the humans they encounter.
W.B. Yeats divided fairies into the solitary and trooping fairies, as did James Macdougall in Folk Tales and Fairy Lore. Katharine Mary Briggs noted that a third distinction might be needed for "domesticated fairies" who live in human households.
These fairies contain the aristocracy of the fairy world, including the Irish Aos Sí. They are known as trooping faeries because they travel in long processions, such as the one from which Tam Lin was rescued. But the trooping fairies also include other fairies of lesser importance; a trooping fairy can be large or small, friendly or sinister.
These fairies generally function as pets or servants of the households they live in, but sometimes might temporarily join with other fairies for merry-making activities.
The folklorist Wirt Sikes formally divided Welsh fairies, or Tylwyth Teg, into five general types. They include the Ellyllon (elves), the Bwbachod (household spirits similar to brownies and hobgoblins), the Coblynau (spirits of the mines), the Gwragedd Annwn (lake maidens), and the Gwyllion (mountain spirits resembling hags). Sikes acknowledged that while such classifications are largely arbitrary, "the student of folklore must classify his materials distinctly in some understandable fashion, or go daft." 
- A buttery spirit haunts the butteries and pantries of dishonest inns and taverns, eating and drinking everything in sight and causing financial misfortune. The only way to get rid of the buttery spirit is for the owner of the establishment to become honest and ethical in his or her business practices. An early account was given by Thomas Heywood in Hierarchie of the Blessed Angels (1635) and reprinted by Scott and Briggs.
- "SND: Seil". Dsl.ac.uk. Archived from the original on 2014-05-13. Retrieved 2014-05-11.
- "SND: Wicht". Dsl.ac.uk. Archived from the original on 2014-05-13. Retrieved 2014-05-11.
- Celtic folklore: Welsh and Manx, Volume 1, John Rhys, Forgotten Books, 1983, ISBN 1-60506-170-0, ISBN 978-1-60506-170-2. pp. 469-470
- Briggs, Katharine Mary (1976) 'An Encyclopedia of Fairies, Hobgoblins, Brownies, Boogies, and Other Supernatural Creatures. New York, Pantheon Books. ISBN 0-394-73467-X
- Silver, Carole G. (1999) Strange and Secret Peoples: Fairies and Victorian Consciousness. Oxford University Press. p.174 ISBN 0-19-512199-6
- Scott, Walter (1849). Minstrelsy of the Scottish Border (Vol. 2). Robert Cadell, Edinburgh. pp. 343–4.
- Briggs (1976), pp. 54–5.
- Merriam-Webster's encyclopedia of literature, Merriam-Webster, 1995, ISBN 0-87779-042-6, ISBN 978-0-87779- 042-6. p.371
- Briggs, K.M. (1967) The Fairies in English Tradition and Literature. Chicago, University of Chicago Press.
- Evans-Wentz, W. Y. (1966, 1990) The Fairy-Faith in Celtic Countries. New York, Citadel p.167 ISBN 0-8065-1160-5
- Francis James Child, The English and Scottish Popular Ballads
- Sikes, Wirt (1880). British Goblins: Welsh Folklore, Fairy Mythology, Legends and Traditions. Sampson Low, Marston, Searle & Rivington.