And I thought it was just made up for the book!
(I’ve been on a Terry Pratchett reading-a-thon lately, and as noted on Kim’s blog here, if you are looking for a great series of books to recommend to young girls, I can’t say enough good things about The Wee Free Men and its sequels).
So, in looking up the link, I discover:
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 specialised traditional number systems was common among shepherds, especially in the dales of the Lake District. The Yan Tan Tethera system was also used for counting stitches in knitting.
As you’ll see from the charts there, the system is rather complex, and I don’t think we’ll be saying “Knit methera-dick, purl tyan, knit methera-dick, cable right front” any time soon, but it’s an interesting bit of knitting-related history for you.