The triquetra is a symmetrical figure of three interlaced arcs forming a three-cornered knot, sometimes enclosed in a circle. As ornament it is ancient and widespread, appearing on Germanic and Scandinavian objects and on Insular manuscripts, and like most interlace it carried no fixed doctrinal meaning where it was first drawn. Its best-known early appearances are in Insular Christian art of the early medieval British Isles, in illuminated gospel books such as the Book of Kells, where a triquetra recurs among the trumpet-spirals and interlaced bands of the elaborate monogram page at folio 34 recto, and on carved stone crosses, where triple knotwork was congenial to a Christian culture that could read three-in-one designs in the light of the Trinity, though the artists rarely tell us they intended that.
Manuscript scholars generally treat the figure, in this period, as one interlace unit among many rather than as a captioned symbol set apart for special meaning, a caution worth keeping given how confidently later tradition reads doctrine back into it. Geometrically the shape is also described as three overlapping vesica piscis lens-shapes, the same lens formed by two intersecting circles that underlies a great deal of medieval compass-drawn ornament, which is part of why closely similar interlace patterns recur independently in artistic traditions that had no contact with one another.