In Archaic and Classical Greek (c. 9th century BC to 4th century BC), it represented an aspirated voiceless bilabial plosive ([p?]), 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 that of a voiceless bilabial fricative ([?]), 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⟩.
It may be that phi originated as the letter qoppa and initially represented the sound /k/ before shifting to Classical Greek [p?]. In traditional Greek numerals, phi has a value of 500 () or 500,000 (). The Cyrillic letter Ef (?, ?) descends from phi.
As with other Greek letters, lowercase phi is used as a mathematical or scientific symbol. Some uses, such as the golden ratio, require the old-fashioned 'closed' glyph, which is separately encoded as the Unicode character ϕ GREEK PHI SYMBOL.
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The lowercase letter ? (or often its variant, ?) is often used to represent the following:
The uppercase letter ? is used as a symbol for:
The diameter symbol in engineering, ?, is often erroneously referred to as "phi", and the diameter symbol is sometimes erroneously typeset as ?. This symbol is used to indicate the diameter of a circular section; for example, "?14" means the diameter of the circle is 14 units.
In Unicode, there are multiple forms of the phi letter:
|Character||Name||Correct appearance||Your browser||Usage|
|U+03A6||GREEK CAPITAL LETTER PHI||?||Used in Greek texts|
|U+03C6||GREEK SMALL LETTER PHI||or||?||Used in Greek texts|
|U+03D5||GREEK PHI SYMBOL||? (?)||Used in mathematical and technical contexts. (Italicized.)|
|U+0278||LATIN SMALL LETTER PHI||?||Used in IPA to symbolise a voiceless bilabial fricative|
In ordinary Greek text, the character U+03C6 ? is used exclusively, although this character has considerable glyphic variation, sometimes represented with a glyph more like the representative glyph shown for U+03C6 (?, the "loopy" or "open" form) and less often with a glyph more like the representative glyph shown for U+03D5 (?, 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.
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 LaTeX, the math symbols are \Phi (), \phi (), and \varphi ().
The Unicode standard also includes the following variants of phi and phi-like characters:
|U+1D60||MODIFIER LETTER SMALL GREEK PHI||?|
|U+1D69||GREEK SUBSCRIPT SMALL LETTER PHI||?|
|U+1DB2||MODIFIER LETTER SMALL PHI||?|
|U+2CAA||COPTIC CAPITAL LETTER FI||?|
|U+2CAB||COPTIC SMALL LETTER FI||?|
|U+2C77||LATIN SMALL LETTER TAILLESS PHI||?|
|U+1D6BD||MATHEMATICAL BOLD CAPITAL PHI||?|
|U+1D6D7||MATHEMATICAL BOLD SMALL PHI||?|
|U+1D6DF||MATHEMATICAL BOLD PHI SYMBOL||?|
|U+1D6F7||MATHEMATICAL ITALIC CAPITAL PHI||?|
|U+1D711||MATHEMATICAL ITALIC SMALL PHI||?|
|U+1D719||MATHEMATICAL ITALIC PHI SYMBOL||?|
|U+1D731||MATHEMATICAL BOLD ITALIC CAPITAL PHI||?|
|U+1D74B||MATHEMATICAL BOLD ITALIC SMALL PHI||?|
|U+1D753||MATHEMATICAL BOLD ITALIC PHI SYMBOL||?|
|U+1D76B||MATHEMATICAL SANS-SERIF BOLD CAPITAL PHI||?|
|U+1D785||MATHEMATICAL SANS-SERIF BOLD SMALL PHI||?|
|U+1D78D||MATHEMATICAL SANS-SERIF BOLD PHI SYMBOL||?|
|U+1D7A5||MATHEMATICAL SANS-SERIF BOLD ITALIC CAPITAL PHI||?|
|U+1D7BF||MATHEMATICAL SANS-SERIF BOLD ITALIC SMALL PHI||?|
|U+1D7C7||MATHEMATICAL SANS-SERIF BOLD ITALIC PHI SYMBOL||?|