Voiceless alveolar trill

The voiceless alveolar trill differs from the voiced alveolar trill /r/ only by the vibrations of the vocal cord. 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.

• PIE *srew- > Ancient Greek ῥέω "flow", possibly [r̥é.ɔː]