The early Britons’ survival relied on their livestock. Shared pastures meant you had to know how many you put in (in order to take back out) and taxes were later applied per head of stock. A stock count was the first action performed in the morning and the last at night.
Stock counting systems derived from Brythonic/Celtic languages and practices though the actual words have been greatly corrupted by time. But for ease of purpose (and much like all oral traditions of the time) rhyme and metre were key to remembering the sequence. Thus, the counting system could be used for everything from stock counting to keeping track of stitching when knitting yarn.
Every region had its own rhyming words for the numbers up to twenty and there is a reasonably consistent rhyming pattern across the twenty numbers.
The Borrowdale (Cumberland) dialect is a good example of the Yan Tan Tether counting and rhyming system and has served the shepherds of the lake valleys for two millennia:
Yan, Tyan, Tethera, Methera, Pimp, Sethera, Lethera, Hovera, Dovera, Dick, Yan a dick, Tyan a dick, Tethera dick, Methera dick, Bumfit, Yan a bumfit, Tyan a bumfit, Tethera bumfit, Methera bumfit, Giggot
Even now, people count aloud under their breath when tallying up numbers. In modern English we still rely on alliterated pairs in lieu of rhyme – two/three, four/five, six/seven — and like Yan Tan Tether we use ‘ten and’ system for the higher numbers: three-and-ten, four and ten, five and ten (thirteen, fourteen, fifteen). (in Yan Tan its ‘ten and one’ ‘ten and two’ ten and three’ ‘ten and four’ ‘fifteen’ ‘fifteen-and-one, fifteen-and-two etc)
Here’s the very early origins of our (English) counting system (courtesy of Wikipedia). Not much in common, but then again Celtic languages were a mile from the Germanic language that later became English (check out this great language map)