It occurs in a few languages, usually alongside the voiced version, as a similar phoneme or an allophone.

Proto-Indo-European *sr developed into a sound spelled ⟨ῥ⟩, with the letter for /r/ and the diacritic for /h/, in Ancient Greek.

It was probably a voiceless alveolar trill and became the regular word-initial allophone of /r/ in standard Attic Greek that has disappeared in Modern Greek.

Features of the voiceless alveolar trill: The voiceless alveolar fricative trill is not known to occur as a phoneme in any language, except possibly the East Sakhalin dialect of Nivkh.

Shaded areas denote articulations judged impossible.