Psi (letter) Psi (letter) Psi (; uppercase ', lowercase '; "psi" ) is the 23rd letter of the Greek alphabet and has a numeric value of 700. In both Classical and Modern Greek, the letter indicates the combination /ps/ (as in English word "lapse"). For Greek loanwords in Latin and modern languages with Latin alphabets, psi is usually transliterated as "ps". The letter's origin is uncertain. It may or may not derive from the Phoenician alphabet. It appears in the 7th century BC, expressing /ps/ in the Eastern alphabets, but /kʰ/ in the Western alphabets (the sound expressed by Χ in the
History of the Greek alphabet foreshadowed the development of vowel letters with a limited use of "matres lectionis", that is, consonants that pulled double duty as vowels, which for historical reasons occurred mostly at the ends of words. For example, the two letters "wāw" and "yōdh" stood for both the approximant consonants and , and the long vowels and in Phoenician. By this point in time, Greek had lost its sound, so Phoenician "yōdh" was used only for its vocalic value, becoming the Greek vowel letter iota . However, several Greek dialects still had a sound, and here "wāw" was used for both of its