What the sources claim for it
The four elements and their relation to each other, written so the relation is visible in the sign.
History
Where does the Elemental Triangles come from?
The four signs are built from one shape by two binary choices. Point up is fire, point down is water; add a horizontal bar and fire becomes air, water becomes earth. Four elements, two bits, and the relationships between them readable off the glyphs — a piece of notation that is genuinely well designed.
They belong to alchemical and later ceremonial use rather than to the ancient Greek philosophy they encode, and they attach to a long chain of correspondences: the four qualities, the four humours, the four suits of the tarot in later occult systems, and the four quarters of the Solomonic circle.
Afterwards
What did it turn into?
Carried into modern occult practice as the standard elemental notation, and into popular fantasy and games as generic element icons.
Which texts describe it?
Oldest first, with what each one does with the mark. A mark described in a Bronze Age incantation and a mark first drawn in a seventeenth-century manuscript are different kinds of claim, whatever the shape has in common.
Renaissance & Early-Modern Occult Angelology
Agrippa, Barrett, and their compilers · 16th-19th c. · Latin and English
Agrippa's elemental correspondences are the scheme this notation writes down.
Read Renaissance & Early-Modern Occult Angelology in full ↗Cover: Public domain · Otto Rietmann · Wikimedia file page
The Magus, or Celestial Intelligencer
Francis Barrett · 1801 · English
Barrett reproduces the elemental tables for an English readership.
Read The Magus, or Celestial Intelligencer in full ↗Cover: Public domain · Unknown author · Wikimedia file page
Nearby
Related marks
The Pentagram
A five-pointed star drawn in one unbroken line — an accounting mark, a Pythagorean password, a Christian emblem of the five wounds, and only recently a sign of evil.
The Abraxas Gem
A rooster-headed figure with snakes for legs, cut into jasper by the thousand, and the numerological reason a magic word means 365.
Saturn
The scythe. Slowest planet, heaviest metal, and the authority a grimoire invokes for work involving binding, decay and the dead.
Jupiter
A stylised Greek zeta for Zeus. The sign of increase, favour and judgment in one's favour — the grimoires' preferred authority for asking.
Mars
Shield and spear, iron, Tuesday — and, since Linnaeus, the biological sign for male, which is a much later and quite separate job.
The Sun
A circle with a point at its centre — the oldest form in the set, and the one the alchemists used for gold without further explanation.