Phi ( ; uppercase Φ, lowercase φ or ϕ; ''pheî'' ; Modern Greek: ''fi'' ) is the twenty-first letter of the Greek alphabet. In Archaic and Classical Greek ( 9th to 4th century BC), it represented an aspirated voiceless bilabial plosive (), which was the origin of its usual romanization as . 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 . 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 () or 500,000 (). The Cyrillic letter Ef (Ф, ф) descends from phi. Like other Greek letters, lowercase phi ...