Combining Low Line

Add the mark immediately after each character. In a̲, the line follows a; a̲b̲c̲ places a mark after every letter so the underlines can meet. A mark on its own has no base to attach to, so it may not appear as a useful standalone character.

That makes an underbar part of the text sequence, not a document style. It can supply an underline in a plain-text field that rejects formatting; every character needs its own mark.

Unicode designs low lines to connect to neighbors, but spacing and other combining marks can spoil the join. In a document with formatting, use ordinary styled underlining; this sequence is best kept for plain-text interchange, and its result can vary by font.

What else can a line below a letter mean?

The notation or spelling decides; the mark alone does not. A pronunciation-dictionary example uses an underlined vowel, ɪ̲, to mark stress. APL, a programming language with its own symbols, treats underbarred letters as distinct symbols; Unicode builds an a underbar by putting this mark after the letter.

Some spellings use the line below to change how a letter is read. Arabic romanization guidance allows this longer connecting line under H, S, and s, but recommends macron below when a short separate mark is intended; a North American Indigenous-font project also documents longer line-below letters in Indigenous orthographies.

Unicode's documented background connects line-below notation with IPA, an alphabet for writing speech sounds, and older character-set standards ISO 5426 and ISO 6937. IBM APL code-page materials from 1984 and 1989 include underlined letters, evidence of the mark in APL computing rather than a date for its origin.