The spiritus asper ("rough breathing") is the mark of an initial aspiration in ancient Greek. It is written as an opening half moon on top of or to the left of an initial vowel and rho:
- ἁ- ἑ- ἡ- ἱ- ὁ- ὑ- ὡ-, ῥ-;
- Ἁ- Ἑ- Ἡ- Ἱ- Ὁ- Ὑ- Ὡ, Ῥ-.
- on top of a double rho in certain editions;
- when it represents a coronis resulting from a crasis implying a vowel bearing a spiritus asper.
When a word begins by an initial grapheme which is a vowel not preceeded by an [h], the spiritus lenis ("soft breath") must be employed.