Jason Bellchamber demonstrates how to draw a Celtic double spiral from the Chi-Ro page of the book of Kells.
This pattern is aesthetically the same as the Asian symbol ‘Yin Yang’ but not identical. Celtic sprial designs are likely an abstract representation of balance and predate 5th century knotwork patterns by a millenia. Originally the early metalsmiths and artisans would use spirals as zoomorphic (abstract human or animal forms) and plant motifs in a spiral form. In the Book of Kells (late 8th Century) spiral designs are found as either doubles and most commonly the triple.
A fast motion cut of this vidoe is available here. https://youtu.be/d9tkEktZ4OE
At this point in Celtic art history, animals and human forms begin to look more realistic in design, but it still predates medieval styles by over a century..
This double spiral is elegant, in being 2 equal halves of a whole which suggests a symmetrical relationship. Triple spiral instructions are here http://youtu.be/XX2LDyiYG40
Another video series of the double spiral alternating with a triskele interlace is here http://youtu.be/3nc31YpozJs