Making Celtic Knots (felt-tip pen)

This is an easy way to draw a Celtic knot. You don't have to rub anything out! This method uses colour and outlines. It is particularly suitable for free-form, flowing knots and entangled animals, as used in illuminated manuscripts. It is so simple that it can be used by young children. If drawing it on paper (in the 'real world'), then use a pale broad felt tip pen for the initial lines, and a dark thin felt tip for the outlines.
 Draw some lines Cross with some more lines Outline one line, with a gap where the line goes underneath Outline the other lines. Make sure the gaps are right! Outline the rest of the lines Join two of the lines Join the rest, with the corners This has left two ends. I've added a head and tail!

 Here is an action replay of that!

If you have an even number of lines in both directions, as in the example above, there is only one way to have the knot as a single string, and you will get two loose ends. You can get other effects by joining the ends in different ways, but these lead to different numbers of strings. I've coloured them differently to high-light this, but you can colour them all the same and many people wouldn't notice!
If you want a knot with just one string and no loose ends, you must start with an odd number of lines in both directions. You don't need to restrict yourself to a square. You can also tilt the shape into a diamond. Here you must start with a square.
You can also join the ends to make other combinations, with more strings.

Once you've mastered the technique, you can try more complicated shapes. You can vary the length of the lines to produce a diamond, and then join them in the usual way. The shape on the left uses several strings. The tilted shape on the right only has one string. You might think it similar to the rectangular knots above, but note the diagonal strings.

You can make any shape you want. Draw out the lines in the paler colour, making sure that they cross approximately at right angles, and that you've left a little room between them. Then draw the outlines, making sure that the lines go over and under. You can have circles, if you want! This technique is so simple to do that children under 10 years old can use it to produce their own designs. Just encourage them to draw the crossing lines first, then the outlines, then join the ends, or finish them with twirls or blobs or heads and tails. The octopus on the right is an original design by a child. You didn't know that an octopus could cross its arms, did you?

You do not need to restrict yourself to a simple grid pattern (see left). Draw the crossing lines as you will, then follow one line, outlining as you go.

I am not sure whether the painters of illuminated manuscripts used this technique. I don't see why not. They didn't have felt tip pens, of course, but they could paint in the colour, and let it dry. Then they could do the outlines in a dark paint with a fine brush. This technique couldn't be used in stone, such as the Celtic crosses in the Isle of Man and elsewhere.