Triquetra
A three-cornered interlace knot found across early medieval northern Europe: in Insular Christian manuscripts, on Viking-Age objects, and on Swedish runestones.
The drawing above shows one form of the symbol. Historical examples vary (sometimes considerably), and no single version should be treated as canonical.
Earliest attestation: 7th century
The oldest surviving appearance we can actually point to.
The form appears in Insular Christian art from the 7th century onwards, including the Book of Kells, and on Scandinavian objects and monuments of the Viking Age. On the Uppland runestone U 937 (one of the Funbo family group, though it was found reused in a monastery wall in Uppsala in 1875 and now stands in a park at the university) the runic serpent coils around one as the central display motif. U 484 at Kasby carries another, well formed, in the middle of the stone. The Gotland picture stone Sanda I (G 181) is a looser case: the Gotlandic "triple symbols" resemble the triquetra and may be inspired by Continental use of it, but the relationship is an argument rather than an identification.
Historical use
The triquetra is a decorative interlace motif shared across a wide cultural area rather than a specifically Norse religious sign. Its strongest documented associations are Christian, where it was read as a Trinity symbol.
Modern use
Read today as a triple-goddess or maiden-mother-crone symbol in Wiccan and neopagan practice, and marketed as a Celtic or Viking motif interchangeably. The triple-goddess reading is 20th-century: the maiden, mother and crone triad it rests on was put into circulation by Robert Graves in 1948, and attached to this much older motif later.
Common misconceptions
Claims about this symbol that circulate widely and do not hold up.
The triquetra is not distinctively Norse, and it is not distinctively pagan. Its best-attested medieval use is Christian.
Sources
Full references on the sources page.