U+0331 COMBINING MACRON BELOW

U+0331 was added to Unicode in version 1.1 (1993). It belongs to the block Combining Diacritical Marks in the Basic Multilingual Plane.

This character is a Nonspacing Mark and inherits its script property from the preceding character.

The glyph is not a composition. It has a Ambiguous East Asian Width. In bidirectional context it acts as Nonspacing Mark and is not mirrored. The glyph can, under circumstances, be confused with 3 other glyphs. In text U+0331 behaves as Combining Mark regarding line breaks. It has type Extend for sentence and Extend for word breaks. The Grapheme Cluster Break is Extend.

The Wikipedia has the following information about this codepoint:

Macron below, U+0331 ◌̱ combining macron below (HTML ̱), is a combining diacritical mark used in various orthographies.

It is not to be confused with "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".