Despite its name, U+03D5 GREEK PHI SYMBOL (ϕ) is not used at all for written Greek; instead, U+03C6 GREEK SMALL LETTER PHI (φ) should be used.[1]
In mathematical and technical fields, the two forms are used interchangeably. For example, different authors may variously write the golden ratio with either (\phi in TeX) or (\varphi).
The Unicode 3.0 standard (2002) states: "Fonts used primarily for Greek text may use either glyph form for U+03C6, but fonts that also intend to support technical use of the Greek letters should use the loopy form to ensure appropriate contrast with the straight form used for U+03D5."[1] However, this system was actually reversed prior to Unicode 3.0, meaning that many fonts developed prior to 2002 reverse the glyph shapes of the two characters, so that U+03C6 GREEK SMALL LETTER PHI (φ) is straight and U+03D5 GREEK PHI SYMBOL (ϕ) has a loop.[1]
