Voiceless labiodental fricative in the context of "Φ"

Phi (/ˈfaɪ, ˈfiː/ FY, FEE; uppercase Φ, lowercase φ or ϕ; Ancient Greek: ϕεῖ pheî [pʰéî̯]; Modern Greek: φι fi [fi]) is the twenty-first letter of the Greek alphabet.

In Archaic and Classical Greek (c. 9th to 4th century BC), it represented an aspirated voiceless bilabial plosive ([pʰ]), 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 ([ɸ]), and by the Byzantine Greek period (c. 4th century AD to 15th century AD) it developed its modern pronunciation as a voiceless labiodental fricative ([f]).The romanization of the Modern Greek phoneme is therefore usually ⟨f⟩.

An obstruent (/ˈɒbstruənt/ OB-stroo-ənt) is a speech sound such as [k], [d͡ʒ], or [f] that is formed by obstructing airflow. Obstruents contrast with sonorants, which have no such obstruction and so resonate. All obstruents are consonants, but sonorants include vowels as well as consonants.

Pe is the seventeenth letter of the Semitic abjads, including Arabic fāʾ ف‎, Aramaic pē 𐡐, Hebrew pē פ‎, Phoenician pē 𐤐, and Syriac pē ܦ. (in abjadi order). It is related to the Ancient North Arabian 𐪐‎, South Arabian 𐩰, and Ge'ez ፈ.

The original sound value is a voiceless bilabial plosive /p/ and it retains this value in most Semitic languages, except for Arabic, where the sound /p/ changed into the voiceless labiodental fricative /f/, carrying with it the pronunciation of the letter. However, the sound /p/ in Arabic is used in loanwords with the letter pe as an alternative. Under the Persian influence, many Arabic dialects in the Persian Gulf, as well as in Egypt and in some of the Maghreb under the Ottoman influence uses the letter pe to represent the sound /p/ which is missing in Modern Standard Arabic. Not to be confused with the Turned g. The Phoenician letter gave rise to the Greek Pi (Π), Latin P, Glagolitic Ⱂ, and Cyrillic П.