Yan tan tethera

In order to keep accurate records (e.g. of birth and death) and prevent animals from straying, shepherds must perform frequent head-counts of their flocks. Dating back at least to the medieval period, and continuing to the present in some areas like Slaidburn, farms were granted fell rights, allowing them access to common grazing land. To prevent overgrazing, it was vitally necessary for each farm to keep accurate, updated head-counts.

Though fell rights are largely obsolete in modern agriculture, farms are often subsidized and taxed according to the quantity of their sheep. For this reason, accurate counts are still necessary, and must be performed frequently.

Generally, a count is the first action performed in the morning and the last action performed at night. A count is made after moving the sheep from one pasture to another, and after any operation involving the sheep, such as shearing, foot-trimming, mulesing, etc.

Sheep-counting systems ultimately derive from a Celtic language, possibly Welsh, Cumbricor the speech of a Brittish 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. 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).

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.

• John Whitehead - counting sheep the traditional way
• More about usage
• Tim Gay - Rural dialects and surviving Britons