Phi Symbol

Phi's lowercase glyph, φ (U+03C6, GREEK SMALL LETTER PHI), is the 21st letter of the Greek alphabet, but it's most famous as the symbol for the golden ratio — the irrational number (1 + √5) / 2 ≈ 1.6180339887 that artists and architects have chased for centuries. That naming is surprisingly recent: the ratio was long written with the Greek letter tau (τ) until the American mathematician Mark Barr proposed φ around 1909, picking it as the first letter of Phidias (Φειδίας, c. 490–430 BC), the sculptor tied to the Parthenon's proportions. Phi also carries a quirk no other Greek letter here has — two lowercase glyphs. The everyday letter φ (U+03C6) usually renders as the 'loopy,' open form, while a separately encoded Greek phi symbol, ϕ (U+03D5), is the 'closed,' stroked form — a circle crossed by a vertical line — that Unicode has recommended for mathematics since version 3.0, though which shape a font actually shows varies by typeface. The capital Φ (U+03A6) does heavier lifting in the sciences: it's the standard symbol for magnetic flux in physics, while lowercase φ marks Euler's totient function, φ(n), in number theory.