Often I hear the term overline. What is an overline and what is its function and style in typography?
An overline, overscore, or overbar, is a typographical feature of a horizontal line drawn immediately above the text. In mathematical notation, an overline has been used for a long time as a vinculum, a way of showing that certain symbols belong together. The original use in Ancient Greek was to indicate compositions of Greek letters as Greek numerals. In Latin it indicates Roman numerals multiplied by a thousand and it forms medieval abbreviations (sigla). Marking one or more words with a continuous line above the characters is sometimes called overstriking, though overstriking generally refers to printing one character on top of an already-printed character.
An overline, that is, a single line above a chunk of text, should not be confused with the macron, a diacritical mark placed above (or sometimes below) individual letters. The macron is narrower than the character box. Since ISO and Unicode Consortium assign names to characters in their unique fashion and often ignore the established typographical terminology, Unicode includes two characters U+00AF ¯ MACRON (formerly spacing macron) and U+203E ‾ OVERLINE that both look like an overlined space in most fonts, similar to a mirrored underscore symbol. An overline proper can be encoded as a Unicode diacritic;
Much like an Underline, an Overline is simply used for emphasis in general typography where mathematics is not a concern. Its simply another way to draw attention to type.