Yan Tan Tethera was a traditional numeric jargon used by shepherds to count sheep in northern England and southern Scotland. Until the Industrial Revolution, the use of specialized traditional number systems was common among shepherds, especially in the dales of the Lake District.
Though these number systems fell out of use by 1910, the word yan remains in use to mean "one" in some northern English dialects. The practice may also have given rise to the notion of counting sheep to lull oneself to sleep.

The importance of keeping count
Sheep-counting systems ultimately derive from a Celtic language, possibly Welsh, Cumbric or the speech of a British population surviving after the Anglo-Saxon conquest. It is impossible, given the corrupted form in which they have survived, for us to be sure of their exact origin.
Like most Celtic numbering systems, they tend to be vigesimal — based on the number twenty. Moreover, they usually lack words to describe quantities larger than twenty (though this is not a limitation of complete Celtic counting systems). To count a large number of sheep, a shepherd would repeatedly count to twenty, placing a mark on the ground to represent each score (e.g. 100 sheep = 5 score sheep).

Origin and usage

Systems by region

Numerals in Brythonic Celtic Languages
The English composer Harrison Birtwistle (b. 1934) composed an opera entitled Yan Tan Tethera (subtitled "a mechanical pastoral") in 1984.
English chansonnier Jake Thackray wrote, performed and recorded a song about a shepherdess, entitled Old Molly Metcalfe, with the refrain Yan Tean Tether Mether Pip she counted. In the introduction to the song he describes how Swaledale sheep farmers "count their sheep in a curious fashion," and gives the entire sequence from 1 to 20.
In Terry Pratchett's novel The Wee Free Men, the heroine, Tiffany Aching, was called Jiggit by her Grandmother, a female shepherd, as Tiffany was her twentieth grandchild.
Yan Tan Tethera is the name of a book by David Herter related to his first novel, Ceres Storm.
In Garth Nix novel Grim Tuesday, Grim Tuesday splits his Dawn, Noon, and Dusk servants into seven parts named Yan, Tan, Tethera, Methera, Pits, Sethera and Azer.
The children's album "Fiddle Up a Tune" by Eric Nagler features a song "Yan Tan Tethera," whose eponymous phrase begins an incantation used to calm leprechauns: "Yan tan tethera, one two three: All you little ones, let us be. Hevapin sethera, four five six: Lay down your magic fiddlesticks."