The Shield Knot is one of the few bits of knot work that actually has a traditional meaning. Though the design varies quite a bit the composition is almost always circular and composed of for parts or petals. It is probably originally a Norse or Pictish design, though, like most knot work, the Celts quickly adopted it. It is almost always used as a symbol of protection and warding. It can be found carved into rune stones, door posts and other everyday items.
St. Hans' Cross is believed to be a variant of a Shield Knot.