Yan Tan Tethera or yan-tan-tethera is a sheep-counting system traditionally used by shepherds in Northern England and some other parts of Britain.^[1] The words are numbers taken from Brythonic Celtic languages such as Cumbric which had died out in most of Northern England by the sixth century, but they were commonly used for sheep counting and counting stitches in knitting until the Industrial Revolution, especially in the fells of the Lake District. Though most of these number systems fell out of use by the turn of the 20th century, some are still in use.

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Sheep-counting systems ultimately derive from Brythonic Celtic languages, such as Cumbric; Tim Gay writes: “[Sheep-counting systems from all over the British Isles] all compared very closely to 18th-century Cornish and modern Welsh".^[2] It is impossible, given the corrupted form in which they have survived, to be sure of their exact origin. The counting systems have changed considerably over time. A particularly common tendency is for certain pairs of adjacent numbers to come to resemble each other by rhyme (notably the words for 1 and 2, 3 and 4, 6 and 7, or 8 and 9). Still, multiples of five tend to be fairly conservative; compare bumfit with Welsh pymtheg, in contrast with standard English fifteen.

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Like most Celtic numbering systems, they tend to be vigesimal (based on the number twenty), but they usually lack words to describe quantities larger than twenty; this is not a limitation of either modernised decimal Celtic counting systems or the older ones. To count a large number of sheep, a shepherd would repeatedly count to twenty, placing a mark on the ground, or move a hand to another mark on a shepherd's crook, or drop a pebble into a pocket to represent each score (e.g. 5 score sheep = 100 sheep).

Their use is also attested in a "knitting song" known to be sung around the middle of the nineteenth century in Wensleydale, Yorkshire, beginning "yahn, tayhn, tether, mether, mimph".^[3]

The counting system has been used for products sold within Northern England, such as prints,^[4] beers,^[5] alcoholic sparkling water (hard seltzer in U.S.),^[6] and yarns,^[7] as well as in artistic works referencing the region, such as Harrison Birtwistle's 1986 opera Yan Tan Tethera.

Jake Thackray's song "Old Molly Metcalfe"^[8] from his 1972 album Bantam Cock uses the Swaledale "Yan Tan Tether Mether Pip" as a repeating lyrical theme.

The word yan or yen for 'one' in Cumbrian, Northumbrian, and some Yorkshire dialects generally represents a regular development in Northern English in which the Old English long vowel /ɑː/ <ā> was broken into /ie/, /ia/ and so on. This explains the shift to yan and ane from the Old English ān, which is itself derived from the Proto-Germanic *ainaz.^[9][10] Another example of this development is the Northern English word for 'home', hame, which has forms such as hyem, yem and yam all deriving from the Old English hām.^[11]

• Counting-out game