**Phi** (/faɪ/;^{[1]} uppercase **Φ**, lowercase **φ** or **ϕ**; Ancient Greek: ϕεῖ *pheî* [pʰé͜e]; Greek: φι *fi* [fi]) 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].^{[2]} In traditional Greek numerals, phi has a value of 500 (φʹ) or 500,000 (͵φ). The Cyrillic letter Ef (Ф, ф) descends from phi.

Phi is also used as a symbol for the golden ratio and on other occasions in math and science. This use is separately encoded as the Unicode glyph **ϕ**. The modern Greek pronunciation of the letter is sometimes encountered in English (as /fiː/) when the letter is being used in this sense.^{[3]}

## Use as a symbol

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The lowercase letter φ (or often its variant, ϕ) is often used to represent the following:

- Magnetic flux in physics
- The letter psi is commonly used in physics to represent wave functions in quantum mechanics, such as in the Schrödinger equation and bra���ket notation: .
- The golden ratio 033988749894848204586834... in mathematics, art, and architecture. 1.618
- Euler's totient function
*φ*(*n*) in number theory; also called*Euler's phi function*. - The cyclotomic polynomial functions Φ
_{n}(*x*) of algebra. - In algebra, group or ring homomorphisms
- In probability theory,
*ϕ*(*x*) = (2π)^{−½}*e*^{−x²/2}is the probability density function of the normal distribution. - In probability theory,
*φ*_{X}(*t*) = E[*e*^{itX}] is the characteristic function of a random variable*X*. - An angle, typically the second angle mentioned, after
*θ*(theta). Especially:- The argument of a complex number.
- The phase of a wave in signal processing.
- In spherical coordinates, mathematicians usually refer to phi as the polar angle (from the
*z*-axis). The convention in physics is to use phi as the azimuthal angle (from the*x*-axis). - One of the dihedral angles in the backbones of proteins in a Ramachandran plot
- Internal or effective angle of friction.

- The work function of a surface, in solid-state physics.
- A shorthand representation for an aromatic functional group in organic chemistry.
- The fugacity coefficient in thermodynamics.
- The ratio of free energy destabilizations of protein mutants in phi value analysis.
- In cartography, geodesy and navigation, latitude.
- In aircraft flight mechanics as the symbol for bank angle (sometimes represented with the letter theta, which is also used for pitch angle).
- In combustion engineering, fuel–air equivalence ratio. The ratio between the actual fuel air ratio to the stoichiometric fuel air ratio.
- A sentence in first-order logic.
- The Veblen function in set theory
- Porosity in geology and hydrology.
- Strength (or resistance) reduction factor in structural engineering, used to account for statistical variabilities in materials and construction methods.
- The symbol for a voiceless bilabial fricative in the International Phonetic Alphabet (using the straight line variant character)
- In flight dynamics, the roll angle.
- In philosophy, φ is often used as shorthand for a generic act. (Also in uppercase.)
^{[citation needed]} - In perceptual psychology, the phi phenomenon is the apparent motion caused by the successive viewing of stationary objects, such as the frames of a motion picture.
- In lexical-functional grammar, the function that maps elements from the c-structure to the f-structure.
- In ecology, site survival probability, or the probability that a species will continue to occupy a site if it was there the previous year.
- The logo of La France Insoumise, a leftist French political party.

The uppercase letter Φ is used as a symbol for:

- The golden ratio conjugate −0.618... in mathematics.
- The magnetic flux and electric flux in physics, with subscripts distinguishing the two.
- The cumulative distribution function of the normal distribution in mathematics and statistics.
- In philosophy, Φ is often used as shorthand for a generic act. (Also in lowercase.)
^{[citation needed]} - The number of phases in a power system in electrical engineering, for example 1Φ for single phase, 3Φ for three phase.
- A common symbol for the parametrization of a surface in vector calculus.
- In Lacanian algebra, Φ stands for the imaginary phallus and also represents phallic signification; -Φ stands in for castration.
^{[4]}^{[dubious – discuss]}

The diameter symbol in engineering, ⌀, is often erroneously referred to as "phi". This symbol is used to indicate the diameter of a circular section; for example, "⌀14" means the diameter of the circle is 14 units.

## Computing

In Unicode, there are multiple forms of the phi letter:

Character | Name | Correct appearance | Your browser | Usage |
---|---|---|---|---|

U+03A6 | GREEK CAPITAL LETTER PHI | Φ | Used in Greek texts | |

U+03C6 | GREEK SMALL LETTER PHI | or | φ | Used in Greek texts |

U+03D5 | GREEK PHI SYMBOL | ϕ (ϕ) |
Used in mathematical and technical contexts.^{[5]} (Italicized.)
| |

U+0278 | LATIN SMALL LETTER PHI | ɸ | Used in IPA to symbolise a voiceless bilabial fricative |

In ordinary Greek text, the character U+03C6 φ is used exclusively, although this character has considerable glyphic variation, sometimes represented with a glyph more like the representative glyph shown for U+03C6 (φ, the "loopy" or "open" form) and less often with a glyph more like the representative glyph shown for U+03D5 (ϕ, the "stroked" or "closed" form). Unicode makes an effort to distinguish the two by generally calling the loopy form "small letter phi" or "small phi", and by calling the stroked form "phi symbol", but this isn't exclusively true on all variants.

Because Unicode represents a character in an abstract way, the choice between glyphs is purely a matter of font design. While some Greek typefaces, most notably in the Porson family (used widely in editions of classical Greek texts), have a "stroked" glyph in this position (), most other typefaces have "loopy" glyphs. This goes for the "Didot" (or "*apla*") typefaces employed in most Greek book printing (), as well as for the "Neohellenic" typeface often used for ancient texts ().

It is necessary to have the stroked glyph available for some mathematical uses, and U+03D5 GREEK PHI SYMBOL is designed for this function. Prior to Unicode version 3.0 (1998), the glyph assignments in the Unicode code charts were the reverse, and thus older fonts may still show a loopy form at U+03D5.^{[5]}

For use as a phonetic symbol in IPA, Unicode has a separate code point U+0278, LATIN SMALL LETTER PHI, because in this use only the stroked glyph is considered correct. It typically appears in a form adapted to a Latin typographic environment, with a more upright shape than normal Greek letters and with serifs at the top and bottom.

In HTML/XHTML, the upper- and lowercase phi character entity references are `Φ` (Φ) and `φ` (φ), respectively.

In LaTeX, the math symbols are `\Phi` (), `\phi` (), and `\varphi` ().

The Unicode standard also includes the following variants of phi and phi-like characters:

Character | Name | Appearance |
---|---|---|

U+1D60 | MODIFIER LETTER SMALL GREEK PHI | ᵠ |

U+1D69 | GREEK SUBSCRIPT SMALL LETTER PHI | ᵩ |

U+1DB2 | MODIFIER LETTER SMALL PHI | ᶲ |

U+2CAA | COPTIC CAPITAL LETTER FI | Ⲫ |

U+2CAB | COPTIC SMALL LETTER FI | ⲫ |

U+2C77 | LATIN SMALL LETTER TAILLESS PHI | ⱷ |

U+1D6BD | MATHEMATICAL BOLD CAPITAL PHI | 𝚽 |

U+1D6D7 | MATHEMATICAL BOLD SMALL PHI | 𝛗 |

U+1D6DF | MATHEMATICAL BOLD PHI SYMBOL | 𝛟 |

U+1D6F7 | MATHEMATICAL ITALIC CAPITAL PHI | 𝛷 |

U+1D711 | MATHEMATICAL ITALIC SMALL PHI | 𝜑 |

U+1D719 | MATHEMATICAL ITALIC PHI SYMBOL | 𝜙 |

U+1D731 | MATHEMATICAL BOLD ITALIC CAPITAL PHI | 𝜱 |

U+1D74B | MATHEMATICAL BOLD ITALIC SMALL PHI | 𝝋 |

U+1D753 | MATHEMATICAL BOLD ITALIC PHI SYMBOL | 𝝓 |

U+1D76B | MATHEMATICAL SANS-SERIF BOLD CAPITAL PHI | 𝝫 |

U+1D785 | MATHEMATICAL SANS-SERIF BOLD SMALL PHI | 𝞅 |

U+1D78D | MATHEMATICAL SANS-SERIF BOLD PHI SYMBOL | 𝞍 |

U+1D7A5 | MATHEMATICAL SANS-SERIF BOLD ITALIC CAPITAL PHI | 𝞥 |

U+1D7BF | MATHEMATICAL SANS-SERIF BOLD ITALIC SMALL PHI | 𝞿 |

U+1D7C7 | MATHEMATICAL SANS-SERIF BOLD ITALIC PHI SYMBOL | 𝟇 |

## See also

Look up , Φ, or φ in Wiktionary, the free dictionary.phi |

- F, f: Ef (Latin)
- Ф, ф: Ef (Cyrillic)
- 中
- Psi and phi type figurine
- Փ (Armenian letter)
- Deposition (geology)

## References

**^***Oxford English Dictionary*, 3rd ed. "phi,*n.*" Oxford University Press (Oxford), 2005.**^**Brixhe, C. "History of the Alphabet", in Christidēs & al.'s*A History of Ancient Greek*. 2007.**^**See, e.g.,*The Da Vinci Code*and the*Criminal Minds*episode, "Masterpiece".**^**Evans, Dylans (1996).*An introductory dictionary of Lacanian psychoanalysis*. Routledge. p. 145. ISBN 978-0-415-13523-8.- ^
^{a}^{b}"Representative Glyphs for Greek Phi".*UTR #25: Unicode support for mathematics*(PDF).