Spiritus lenis

The spiritus lenis ("soft breathing") is the mark of the absence of initial aspiration in ancient Greek. Some think it signifies the glottal stop some languages use to avoid initial vowel sounds. It is written as a closing half moon on top of or to the left of an initial vowel:

• ἀ- ἐ- ἠ- ἰ- ὀ- ὐ- ὠ ;
• Ἀ- Ἐ- Ἠ- Ἰ- Ὀ- ὘- Ὠ.