Phi (uppercase Φ, lowercase φ or ϕ; pheî ; Modern Greek: φι fi ) is the twenty-first number of the Greek alphabet. In Archaic and Classical Greek (c. 9th to 4th century BC), it represented an aspirated voiceless bilabial plosive, which was the origin of its usual romanization as ⟨ph⟩. During the later part of Classical Antiquity, in Koine Greek (c. 4th century BC to 4th century AD), its pronunciation shifted to a voiceless bilabial fricative, and by the Byzantine Greek period (c. 4th century AD to 15th century AD) it developed its modern pronunciation as a voiceless labiodental fricative. The romanization of the Modern Greek phoneme is therefore usually ⟨f⟩. It may be that phi originated as the letter qoppa (Ϙ, ϙ), and initially represented the sound before shifting to Classical Greek. In traditional Greek numerals, phi has a value of 500 (φʹ) or 500,000 (͵φ). The Cyrillic letter Ef (Ф, ф) descends from phi. Like other Greek letters, lowercase phi (encoded as the Unicode character ) is used as a mathematical or scientific symbol. Some uses require the old-fashioned 'closed' glyph, which is separately encoded as the Unicode character.

Use as a symbol

In lowercase

The lowercase letter φ (or its variant, ϕ or ɸ) is often used to represent the following:

In uppercase

The uppercase Φ is used as a symbol for:

Unicode

In Unicode, there are multiple forms of the phi letter: In ordinary Greek text, the character U+03C6 φ is used exclusively, though this character has considerable glyphic variation, sometimes represented with a glyph more like the representative glyph shown for U+03C6 (φ, the "loopy" or "open" form), and less often with a glyph more like the representative glyph shown for U+03D5 (ϕ, the "stroked" or "closed" form). Because Unicode represents a character in an abstract way, the choice between glyphs is purely a matter of font design. While some Greek typefaces, most notably those in the Porson family (used widely in editions of classical Greek texts), have a "stroked" glyph in this position, most other typefaces have "loopy" glyphs. This also applies to the "Didot" (or "apla") typefaces employed in most Greek book printing, as well as the "Neohellenic" typeface often used for ancient texts. It is necessary to have the stroked glyph available for some mathematical uses, and U+03D5 GREEK PHI SYMBOL is designed for this function. Prior to Unicode version 3.0 (1998), the glyph assignments in the Unicode code charts were the reverse, and thus older fonts may still show a loopy form \varphi at U+03D5. For use as a phonetic symbol in IPA, Unicode has a separate code point U+0278, LATIN SMALL LETTER PHI, because only the stroked glyph is considered correct in this use. It typically appears in a form adapted to a Latin typographic environment, with a more upright shape than normal Greek letters and with serifs at the top and bottom. In LaTeX, the math symbols are (\Phi,!), (\phi,!), and (\varphi,!). The Unicode standard includes the following variants of phi and phi-like characters: