Combining macron below

Its job depends on the writing system. A bar under a letter can mark a sound contrast, a transliteration choice (writing a language in another alphabet), or a language-specific letter; it has no universal pronunciation.

In Comanche, whose alphabet was adopted in 1994, a bar under a vowel marks it as voiceless or whispered. In Patani Malay written with Thai script, a bar below a consonant marks a sound distinction from Thai spelling. These are separate conventions, not competing definitions.

How do I type a combining macron below?

Type the base letter—the letter being marked—first, then add this mark. A combining mark needs a base: type a followed by ̱ to produce a̱.

On its own, it may look blank or appear beside a dotted placeholder; text tools can count it separately from the visible letter. If the bar sits too high, too low, or detached, test the font: descenders and existing underlines can crowd it.

In Patani Malay written with Thai script, a lower vowel sign may follow the marked consonant. Text normalization, which can reorder equivalent marks in stored text, may change their sequence; the font still needs to stack them as intended.

Is a macron below the same as an underline?

No: this mark attaches to one base letter. Compare a̱ḇ with a̲b̲: this mark sits under each letter, while the combining low line can join beneath neighboring letters to form a continuous line. A word processor's underline is formatting applied to text, not a character in it.