The fairies of Irish, English and Scottish 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 (Dökkálfar and Ljósálfar), 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) 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).
The Northern and Middle English word seely (also seily, seelie and sealy), 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 being 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
The categorization of fairies based on court is whether or not a fairy is light or dark. The Seelie court are known to seek help from humans, to warn those who have accidentally offended them, and to return human kindness with favors of their own. Still, a fairy belonging to this court will avenge insults and could be prone to mischief. The most common time of day to see them is twilight. Other names for the Seelie court are 'The Shining Throne' or 'The Golden Ones' and 'The Summer Court'. Seelies are known for playing pranks on humans and having a light hearted attitude, forgetting their sorrows quickly and not realizing how they might be affecting the humans they play pranks on.
The Unseelie Court consists of the darkly-inclined fairies. Unlike the Seelie Court, no offense is necessary to bring down their assaults. As a group (or "host"), they 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. Like the beings of the Seelie Court who are not always benevolent, neither are the fairies of the Unseelie Court always malevolent. Most Unseelies can become fond of a particular human if they are viewed as respectful, and would choose to make them something of a pet. Some of the most common characters in the Unseelie Court are Bogies, Bogles, Boggarts, Abbey Lubbers and Buttery Spirits. The division into "Seelie" and "Unseelie" spirits was roughly equivalent to the division of Elves in Norse mythology, into "light" and "dark" distinctions.
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.
The Welsh fairies, Tylwyth Teg, and the Irish Aos Sí are usually not classified as wholly good or wholly evil.
Trooping and Solitary Fairies
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, but such fairies might join with other fairies for merry-making and fairs.
The trooping 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.
Unlike the trooping fairies, solitary fairies live alone and are inclined to be wicked and malicious creatures, except for beings such as the brownie who is said to help with household chores.