The Book of Kells is a Bible that was created in Ireland in 800 a.d. The unique aspect of this book is that it included Celtic knots as part of the decoration of the manuscript:

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How to create a Celtic Knot with rope:

How to draw a Celtic Knot:

You can print out graph paper here:
This allows you to make a knot on paper.

Celtic knots, fractals, and some types of tessellations share interesting features. There is regularity within them, so they are recognizable as patterns, but there is also asymmetry that takes them out of Euclidian forms (squares, spheres, and so forth). Here is an investigation into Celtic knots, tessellations, and the use of Inkscape for creating such things.

For a more accurate knot, you can use a computer program.

When you create something in Inkscape, you can take that image and make it very large or very small without it becoming pixellated:

Here is a bitmap image, a jpg file:
Let's make it bigger:
See how the edges are getting blurry? Now here is a more or less faithful copy of that letter in Inkscape:

This image is a jpg file (part of the problem), so it won't scale. But within Inkscape I can scale it without losing accuracy:

See how clear it is? Beautiful!! Inkscape produces "scalable vector graphics" files (".svg") which allow you to create something very accurately. (Notice the Celtic knot in the middle of this?)

Here is a Celtic knot made using the instructions referenced above, but created in Inkscape:
Inkscape allows you to customize the background grid. That means I didn't have to draw the graph lines and that they disappear when I turn off the grid. I also used guidelines.

Basic resources about Celtic Knots

pdf on knots