Phi (uppercase
Φ, lowercase or ; ,
pheî, ; modern ,
fi, ; English: ) is the 21st letter of the
Greek alphabet. In
Ancient Greek, it represented an
aspirated voiceless bilabial plosive , which was the origin of its usual romanization as "ph". In modern Greek, it represents a
voiceless labiodental fricative and is correspondingly romanized as "f". Its origin is uncertain but it may be that phi originated as the letter
qoppa and initially represented the sound before shifting to Classical Greek . In traditional
Greek numerals, phi has a value of 500 or . The Cyrillic letter
Ef (Ф, ф) descends from phi.
