The "line below" diacritic is used in African and native North American languages. It has the 0332 codepoint. Also called underline, it must not be confused with the typographical underline feature.
Questions this diacritic causes:
1. Must this diacritic match the base letter width? I found three approaches so far: (a) fonts with line below matching the base letter width; (b) fonts where the line below has always the same width; and (c) fonts with line below matching thin letters (as i and l) but with a fixed size for the others.
2. If the line below must follow the base width, how to distinguish i or l with line below from i or l with macron below?
3. When pairing two letters with line below, must the diacritics touch each other in order to produce a continuous line?
Similar issues could be argued about the line above diacritic (Unicode 0305)