Phi (uppercase Φ, lowercase or ; Ancient Greek: ϕεῖ, pheî, [pʰé͜e]; modern Greek:φι, fi, [fi]; English: /faɪ/) is the 21st letter of the Greek alphabet. In Ancient Greek, it represented an aspirated voiceless bilabial plosive ([pʰ]), which was the origin of its usual romanization as "ph". In modern Greek, it represents a voiceless labiodental fricative ([f]) 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 /kʷʰ/ before shifting to Classical Greek [pʰ]. In traditional Greek numerals, phi has a value of 500 (φʹ) or 500000 (͵φ). The Cyrillic letter Ef (Ф, ф) descends from phi.