Macrons

Macrons are diacritical marks consisting of a horizontal line placed above a letter. They signal that the vowel is long in many orthographies, or that a vowel's length is phonemically distinctive in linguistic transcription. In writing, macrons appear as precomposed letters such as ā, ē, ī, ō, ū, or as combining diacritics on base letters.

The most well-known historical use is in Latin, where macrons indicated long vowels, a distinction important for meter and for determining word meaning in some forms. In modern orthographies, macrons are used to mark vowel length to help pronunciation and disambiguate meaning in languages such as Māori and Hawaiian (in Hawaiian commonly referred to as kahakō).

Macrons also appear in romanization schemes for languages with long vowels, notably Japanese in Hepburn style (ō, ū). In computational contexts, macrons are encoded in Unicode both as precomposed letters (Ā, ā, Ē, ē, Ī, ī, Ō, ō, Ū, ū) and as the combining macron (U+0304). Some fonts and fonts rendering systems also support the spacing macron character U+00AF for non-combining use.

Summary: A macron marks vowel length; its use varies by language—from historical Latin texts to contemporary orthographies—while in computing it is a standard diacritic supported by Unicode.