Macron below, U+0331 ◌̱ combining macron below (HTML:
̱), is a combining diacritical mark used in various orthographies.
It is not to be confused are "combining minus below" ̠ (U+0320), "combining low line" ̲̲ (U+0332) and "low line" ("underscore") _ ). The difference between "macron below" and "low line" is that the latter will result in an unbroken underline when run together, compare a̱ḇc̱ vs. a̲b̲c̲ (of which only the latter should look like abc).
Note that the Unicode names of composed characters whose decompositions contain this character misleadingly have "line below" rather than "macron below". Thus, ḇ "Latin small letter b with line below" decomposes to "Latin small letter b" and "combining macron below".
Precomposed characters 
Unicode has a number of precomposed characters with "macron below".
- Ḇḇ, Ḏḏ, ẖ, Ḵḵ, Ḻḻ, Ṉṉ, Ṟṟ, Ṯṯ.
- Ḇ (minuscule: ḇ) is used, for example, in the transliteration of Biblical Hebrew into the Roman alphabet to show the fricative value of the letter beth (ב). representing [v], or perhaps[β]. In Unicode the capital Ḇ is codepoint U+1E06 (7686) and the lower case ḇ is U+1E07 (7687).
- Ḏ (minuscule: ḏ) is used, for example, in the transliteration of Biblical Hebrew and Arabic into the Roman alphabet to show the fricative value of the letter dalet (ד), [ð], and in the romanization of Pashto, it is used sometimes to represent retroflex D.
- ẖ is sometimes used for Arabic خ ẖāʼ, Hebrew Heth (letter), 𓄡 second-uniliteral-ẖ Egyptian hieroglyph. Unicode ẖ U+1E96 "Latin small letter h with line below" is canonically equivalent to ẖ (U+0068 U+0331). Note that Unicode does not reserve a codepoint for a "Latin capital letter H with line below". Therefore, a sequence with a combining diacritic "macron below" must be employed: H̱ (U+0048 U+0331).
- Ḻ (minuscule: ḻ) is one possible transliteration of the Tamil letter ழ. Ḻ is used in the Seri language to represent [l], like English l, while unmodified "l" represents [ɬ], like Welsh ll. It is also used in the proposed Unified Alphabet for Mapudungun language.
- Ṉ (minuscule: ṉ) is used in Pitjantjatjara to represent [ɳ], and in Saanich to represent both plain and glottalized [ɴ]. In the romanization of Pashto, it is used sometimes to represent retroflex N.
- Ṯ (minuscule: ṯ) is used in the proposed Unified Alphabet for Mapudungun language representing [t̪]. In the romanization of Pashto, it is used sometimes to represent retroflex T.
- Ṟ (minuscule: ṟ) is sometimes used in the romanization of Pashto to represent the retroflex R.
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