The triquetra is a triangular figure composed of three interlaced arcs, or (equivalently) three overlapping vesicae piscis lens shapes. It is used as an ornamental design in architecture, and in medieval manuscript illumination. Its depiction as interlaced is common in Insular ornaments from about the 7th century. In this interpretation, the triquetra represents the topologically simplest possible knot.

The vesica piscis is a type of lens, a mathematical shape formed by the intersection of two disks with the same radius, intersecting in such a way that the center of each disk lies on the perimeter of the other. In Latin, "vesica piscis" literally means "bladder of a fish", reflecting the shape's resemblance to the conjoined dual air bladders found in most fish. In Italian, the shape's name is mandorla ("almond"). A similar shape in three dimensions is the lemon.