Smooth Breathing

The smooth breathing (Ancient Greek: ψιλὸν πνεῦμα psilòn pneûma; Modern Greek: ψιλή psilí ; Latin: spīritus lēnis) is a diacritical mark used in polytonic orthography. In ancient Greek, it marks the absence of the voiceless glottal fricative /h/ from the beginning of a word.

Some authorities have interpreted it as representing a glottal stop, but a final vowel at the end of a word is regularly elided (removed) where the following word starts with a vowel — and elision would not happen if the second word began with a glottal stop (or any form of stop consonant). In his Vox Graeca, W. Sidney Allen accordingly regards the glottal stop interpretation as "highly improbable".

The smooth breathing ( ᾿ ) is written as on top of one initial vowel, on top of the second vowel of a diphthong, or to the left of a capital, and also in certain editions on the first of a pair of rhos. It did not occur on an initial upsilon, which always has rough breathing (thus the early name ὕ hy, rather than ὔ y).

The smooth breathing was kept in the traditional polytonic orthography even after the /h/ sound had disappeared from the language in Hellenistic times. It has been dropped in the modern monotonic orthography.