In the chronology of the Celtic ornamental symbols, the knotwork design thought to many to be the essence of Celtic art, was actually the last to make an appearance. Scholars believe that the first cross-slabs to appear with multi-strand interlacing date from around the 7th century AD in Scotland and Ireland, the most famous of them being from Cardonagh and Fahan Mura. Through the centuries, the Celtic crosses and stone slabs became decorated with very complex interlaced patterns and other equally intricate decoration, and were adorned with brightly coloured paintings.
The historical design of Celtic knotwork always followed the rules of a continuous ribbon of knot. The line of the knotwork had to have the ability to expand or contract, like liquid filling a designated passage, by adapting itself through necessary changes in its pattern. Celtic scribes had the skill to take the basic design and give it new dimensions of intricacy with practice over time.