Valknut
Three interlocking triangles found on Viking-Age picture stones and grave goods, usually in scenes involving death. Genuinely old, but every name we have for it is modern.
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: 8th–9th century
The oldest surviving appearance we can actually point to.
The symbol appears on the Gotland picture stones Stora Hammars I and Tängelgårda I, on a bed-post from the Oseberg ship burial, and on the Nene River ring from England, though not all four draw it the same way. Stora Hammars I, the Oseberg bed-post and the Nene ring carry the tricursal form, three separate triangles interlocked; Tängelgårda I carries the unicursal one, a single line closed into a trefoil knot. Both are called valknut, and charts that show only the tricursal shape are showing three of the four. The Oseberg burial is dendrochronologically dated to c. 834 and is the firm point here. The picture stones are dated by typology rather than by anything absolute (both belong to Lindqvist's later group, conventionally placed from about 700 onwards), so the century given for them is a bracket rather than a date. This is a Viking-Age symbol either way, unlike several others in this section.
Historical use
Its surviving occurrences cluster around death: ship burials, memorial stones, and scenes that appear to show sacrifice or the passage of the dead. That distribution is the basis for connecting the symbol with Óðinn, who receives the slain. The connection is an inference from context, not something any source states.
Modern use
Widely used today in heathen and Ásatrú practice, in Norse-themed jewellery and tattoo work, and as a general marker of Odinic or warrior symbolism. It has also been adopted by some white-nationalist groups, which the Anti-Defamation League notes in its hate-symbols database, so context matters.
Common misconceptions
Claims about this symbol that circulate widely and do not hold up.
No medieval source records a name for this symbol. "Valknut" is a modern label: the Norwegian word valknute is older, but it denotes a quite different looped knot (the shape English calls St John's Arms), and applying it to these three triangles goes back to Gutorm Gjessing's 1943 paper on the horse in prehistoric art and cult.
The popular gloss "knot of the slain" (valr, "the slain", plus knute, "knot") is one reading of a modern word, not a medieval meaning. Others derive the first element from valk, "something rounded", which would make the name purely descriptive of the shape and sever the connection with death entirely.
The other name in circulation, "Hrungnir's heart", fails in a different way from "valknut". The phrase is medieval, and Snorri goes further than he is usually given credit for: Skáldskaparmál says the giant Hrungnir had a heart of hard stone, spiked with three corners, "just as the carved device since made that is called Hrungnir's heart". So a three-cornered carved figure of that name did exist. What is modern is deciding that it is this one. Snorri describes three corners and no more, which fits a plain triangle or a triquetra as readily as three interlocking triangles, and he never points at an object anyone can now identify.
Claims that the valknut represents the nine worlds, the nine-night hanging of Óðinn, or a specific numerological system are modern interpretations with no textual basis.