Click once on any intersection point to make a horizontal cut. The threads rejoin and now run horizontally past the point. Click again to make a vertical cut, making the threads run vertically past the point. Click a third time to restore the point to its original state with the threads running diagonally through it. If you want to draw a smaller knot, just cut around its perimeter.
This program only helps with quite basic rectangular knots. Most importantly, it's really not very good at drawing. For example, long curves don't look nearly as good as hand drawn ones. You also can't do knots that fill non-rectangular shapes such as circles. However, you can get a pretty good idea of what a rectangular knot would look like if drawn well.
There are three standard reference books about drawing Celtic knots. The simplest and most recent is Iain Bain's
(published in 1986, ISBN 0-8069-8638-7). More comprehensive, but less accessible, is his father's book
Celtic Art, the Methods of Construction
by George Bain (published in 1951, ISBN 0-0946-1830-5). The earliest standard reference is
Celtic Art in Pagan and Christian Times
by J. Romilly Allen (published in 1904, Methuen & Co.).
This copy of the program uses a 30 by 20 grid of squares. If your design needs more space, use the
, which has finer threads and a 60 by 40 grid.
There are lots of relevant web sites — Google or live.com can help you find them. It's worth visiting
, who publish a CD-ROM version of the wonderful
Grammar of Ornament
. There's also my own collection of
. This program and its
are available free. Use at your own risk!