Phi (uppercase Φ, lowercase File:Greek Phi normal.svg or File:Greek phi Didot.svg; 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".