Celtic Knot Thingy

Celtic Knot Thingy (CKT) is a program for drawing Celtic knotwork ornaments such as this one:
or this frieze pattern:
or even this thing:
It is written in Tcl/Tk and PostScript, and so is reasonably platform independent (it works under Windows and Linux, should work on Macs and other Unixes as well).

The method of construction is based on:

Cromwell, Peter. Celtic knotwork: mathematical art. Math. Intelligencer 15(1) (1993), 36-47.
CKT is free for noncommercial use. To learn about getting and using CKT, go to the howto page. To read about how it works, try the explanation page.

I'd love to hear feedback; drop me an email if you find CKT fun or useful (or if you find a bug!).