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.
The origin of the sign is thought to be the right-hand half ( ┤ ) of the letter H, which was used in some Greek dialects as [h] while in others it was used for the vowel eta. In medieval and modern script, it takes the form of a closing half moon (reverse C) or a closing single quotation mark:
Smooth breathings were also used in the early Cyrillic and Glagolitic alphabets when writing the Old Church Slavonic language. Today it is used in Church Slavonic according to a simple rule: if a word starts with a vowel, the vowel has a psili over it. From the Russian writing system, it was eliminated by Peter the Great during his alphabet and font-style reform (1707). All other Cyrillic-based modern writing systems are based on the Petrine script, so they have never had the smooth breathing.
In Unicode, the code points assigned to the smooth breathing are U+0313 ◌̓ combining comma above for Greek and U+0486 ◌҆ combining cyrillic psili pneumata for Cyrillic. The pair of space + spiritus lenis is U+1FBF ◌᾿ greek psili.