# Psi (ψι) (Greek)
**Psi** ([Ancient Greek](https://www.edgepedia.org/ancient-greek) ψεῖ, *pseî*; [Modern Greek](https://www.edgepedia.org/modern-greek) ψι) is the twenty-third letter of the [Greek alphabet](https://www.edgepedia.org/greek-alphabet), written Ψ in uppercase and ψ in lowercase, and representing the consonant cluster /ps/ in both Classical and Modern Greek.[1](https://en.wiktionary.org/wiki/%CF%88%CE%B5%E1%BF%96) • [2](https://www.youtube.com/watch?v=eeorKQRMLvQ) In Greek numerals it has the value 700.[2](https://www.youtube.com/watch?v=eeorKQRMLvQ) It is one of the letters the Greeks added to the alphabet they adapted from Phoenician, and its sound value differed between regional ancient alphabets: /ps/ in the eastern group that became standard, but /kh/ in the western group from which the Latin alphabet descended.[3](https://www.repository.cam.ac.uk/items/80cdf413-9da1-48c9-b199-d2b04770cd61)
| Key fact | Detail |
|---|---|
| Position and forms | 23rd letter of the Greek alphabet; uppercase Ψ, lowercase ψ[1](https://en.wiktionary.org/wiki/%CF%88%CE%B5%E1%BF%96) |
| Sound value | /ps/ as in English "lapse" in both Classical and Modern Greek[2](https://www.youtube.com/watch?v=eeorKQRMLvQ) |
| Ancient name | ψεῖ (*pseî*), neuter, indeclinable; alternative form ψῖ (*psî*); Modern Greek ψι[1](https://en.wiktionary.org/wiki/%CF%88%CE%B5%E1%BF%96) |
| Numeral value | 700 in the Greek alphabetic numeral system[2](https://www.youtube.com/watch?v=eeorKQRMLvQ) |
| Status in alphabet history | One of the supplemental letters (with Φ and Χ) added after the adaptation of the Phoenician alphabet[3](https://www.repository.cam.ac.uk/items/80cdf413-9da1-48c9-b199-d2b04770cd61) |
| Regional values | /ps/ in the dark blue alphabets; /kh/ in the red alphabets[3](https://www.repository.cam.ac.uk/items/80cdf413-9da1-48c9-b199-d2b04770cd61) |
| Unicode | U+03A8 GREEK CAPITAL LETTER PSI; U+03C8 GREEK SMALL LETTER PSI[4](https://www.unicode.org/charts/PDF/U0370.pdf) |
## Name and sound
The letter's Ancient Greek name was ψεῖ (*pseî*), also written ψῖ (*psî*), a neuter indeclinable noun; the Modern Greek name is ψι (*psi*).[1](https://en.wiktionary.org/wiki/%CF%88%CE%B5%E1%BF%96) Reconstructed pronunciations of the name run from /psêː/ in fifth-century BCE Attic to /ˈpsi/ in later varieties of the language.[1](https://en.wiktionary.org/wiki/%CF%88%CE%B5%E1%BF%96) The letter itself denotes the cluster /ps/, as at the end of the English word "lapse".[2](https://www.youtube.com/watch?v=eeorKQRMLvQ) In English, the letter's name is pronounced /ˈsaɪ/ or sometimes /ˈpsaɪ/.[2](https://www.youtube.com/watch?v=eeorKQRMLvQ)
For Greek loanwords in Latin and in modern languages using the [Latin alphabet](https://www.edgepedia.org/latin-alphabet), psi is usually transliterated as "ps", which is why English words such as *psychology* and *pseudonym* begin with the silent letters ps.[2](https://www.youtube.com/watch?v=eeorKQRMLvQ)
## Place in the alphabet's history
The Greek alphabet was adapted from a Phoenician (Semitic) source that reached the Aegean by the earlier eighth century BCE, before the earliest known Greek inscriptions of about 760 BCE.[5](https://academic.oup.com/edited-volume/61673/chapter-abstract/548074837?login=false) The letters Υ, Φ, Χ, Ψ, and Ω are all Greek additions to the Semitic model, and the exact origin of the three double-consonant letters Φ, Χ, and Ψ is disputed, though all appear early.[5](https://academic.oup.com/edited-volume/61673/chapter-abstract/548074837?login=false)
**Regional sound values.** Ancient Greek alphabets were not uniform. The group conventionally called the red alphabets associated Χ with the sequence /ks/ and Ψ with /kh/, while the dark blue alphabets associated Χ with /kh/ and Ψ with /ps/.[3](https://www.repository.cam.ac.uk/items/80cdf413-9da1-48c9-b199-d2b04770cd61) The western Greek alphabet, the basis of the Etruscan and Italic alphabets, belonged to the group in which the psi-shaped letter stood for /kh/; the Latin letter X, representing /ks/, is a direct descendant of the chi-shaped letter through these early Western Greek alphabets, possibly by way of Etruscan.[6](https://www.unicodecharacter.org/u%2B03a8) • [11](https://discovery.ucl.ac.uk/id/eprint/10121058/1/Studies_in_the_Etruscan_loanwo.pdf)
**Origin of the letter shape.** A study by Nicholas Zair, a classicist at the [University of Cambridge](https://www.edgepedia.org/university-of-cambridge), published in the journal *Kadmos*, argues that both the psi- and chi-shaped signs were originally variants of the letter κ representing /k/: such variant shapes are attested for /k/ in the [Phoenician alphabet](https://www.edgepedia.org/phoenician-alphabet) and are also used for /k/ in the [Phrygian alphabet](https://www.edgepedia.org/phrygian-alphabet), an early relative of Greek.[3](https://www.repository.cam.ac.uk/items/80cdf413-9da1-48c9-b199-d2b04770cd61) On this reading, the original value of Ψ must have been /kh/, since that follows from using a variant sign for a velar stop to write another velar stop; the /ps/ value of the standard eastern alphabets is then a later reassignment.[3](https://www.repository.cam.ac.uk/items/80cdf413-9da1-48c9-b199-d2b04770cd61)
Local variation went beyond sound values. Some Greek states produced their own additional symbols for sounds not covered by the common alphabet; a psi-shaped graphic had a restricted use in parts of central Greece and the Achaean colonies.[5](https://academic.oup.com/edited-volume/61673/chapter-abstract/548074837?login=false) In some early writing, the sound /ps/ was not written with a single letter at all: the [Open University](https://www.edgepedia.org/open-university)'s introduction to ancient Greek notes that Ψ (psi) was also represented by two letters, that is, as the sequence πσ.[7](https://www.open.edu/openlearn/mod/oucontent/view.php?id=157789§ion=_unit7.1.11)
## Use in science and mathematics
Like other Greek letters, psi serves as a symbol in mathematics, physics, and engineering, commonly standing for wave functions and other variables.[8](https://symbolinside.com/greek-letters-and-symbols-used-in-science-and-engineering/) The pressure unit "psi" (pounds per square inch) is an acronym rather than the Greek letter, though the two are often confused.[9](https://greekalphabet.net/uses)
## Encoding and typography
Unicode encodes the capital letter at U+03A8 (GREEK CAPITAL LETTER PSI) and the small letter at U+03C8 (GREEK SMALL LETTER PSI) in the Greek and Coptic block.[4](https://www.unicode.org/charts/PDF/U0370.pdf) The NIST Guide to the SI lists the proper forms of psi in both roman and italic type, Ψ and ψ in each case, as part of its table of the Greek alphabet for scientific and technical documents.[10](https://www.nist.gov/pml/special-publication-811/nist-guide-si-chapter-10-more-printing-and-using-symbols-and-numbers)
## References
1. [ψεῖ, Wiktionary](https://en.wiktionary.org/wiki/%CF%88%CE%B5%E1%BF%96)
2. [Psi (letter), Audiopedia](https://www.youtube.com/watch?v=eeorKQRMLvQ)
3. [Nicholas Zair, "The origin and value of the Greek letters <χ> and <ψ>", *Kadmos* (De Gruyter), University of Cambridge Repository](https://www.repository.cam.ac.uk/items/80cdf413-9da1-48c9-b199-d2b04770cd61)
4. [The Unicode Standard, Greek and Coptic chart](https://www.unicode.org/charts/PDF/U0370.pdf)
5. ["alphabet, Greek", Oxford Classical Dictionary, Oxford Academic](https://academic.oup.com/edited-volume/61673/chapter-abstract/548074837?login=false)
6. [U+03A8 Greek Capital Letter Psi, Unicodecharacter.org](https://www.unicodecharacter.org/u%2B03a8)
7. [Getting started on ancient Greek, OpenLearn, The Open University](https://www.open.edu/openlearn/mod/oucontent/view.php?id=157789§ion=_unit7.1.11)
8. [Greek Letters and Symbols Used in Science and Engineering, Symbol Inside](https://symbolinside.com/greek-letters-and-symbols-used-in-science-and-engineering/)
9. [Uses of Greek Letters in Mathematics, Science & Engineering, greekalphabet.net](https://greekalphabet.net/uses)
10. [NIST Guide to the SI, Chapter 10: More on Printing and Using Symbols and Numbers in Scientific and Technical Documents](https://www.nist.gov/pml/special-publication-811/nist-guide-si-chapter-10-more-printing-and-using-symbols-and-numbers)
11. [Studies in the Etruscan Loanwo.pdf, discovery.ucl.ac.uk](https://discovery.ucl.ac.uk/id/eprint/10121058/1/Studies_in_the_Etruscan_loanwo.pdf)
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*Topic: Encyclopedia › Society and history › Languages and linguistics › Writing and notation systems › Mediterranean, Italic, and undeciphered ancient scripts*
*Initially written Sep 17, 2026 · Reviewed: Sep 18, 2026; Sep 30, 2026; Oct 25, 2026 · Edited: Sep 18, 2026; Oct 25, 2026 · Last review: Oct 25, 2026*
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