The Celtic shield knot is used for warding and protection. Shield knots have appeared in cultures around the world and have taken a variety of different forms. They are almost univerally square in shape, and the interlacement of the design ranges from simple to complex. In the Celtic version, a series of knots is formed. In artwork from early Mesopotamia, the shield is simply a square with a loop at each of the four corners.
Fans of Celtic artwork occasionally get variations of this piece as tattoos or wear them as talismans of protection. In modern Celtic reconstructionist groups, the shield knot is sometimes invoked as a ward to keep negative energy away. In some traditions, the corners of the knot are meant to represent the four elements of earth, air, fire and water.