The pattern known as Celtic knotwork was used a lot in the Middle Ages in Wales. You can see it on ancient monuments such as crosses. You can also find it in illuminated manuscripts. The pattern looks like ribbons or ropes that go under and over like a plait of hair. It has hundreds of variations. Usually, the knot has no ends, so you can follow the ribbon round and round.
The Cross of Conbelin is an example of knotwork patterns carved in stone. It is from Margam near Swansea and is a thousand years old. It is a cross placed on a circle.