Voiceless alveolar trill

Some languages possess a voiceless alveolar trill, which differs only in the vibrations of the vocal cord. This is rare, and usually occurs alongside the voiced version as a similar phoneme or an allophone.

Proto-Indo-European developed into a sound spelled $⟨ῥ⟩$, with the letter for  and the diacritic for, in Ancient Greek. It was probably a voiceless alveolar trill, and became the regular word-initial allophone of in standard Attic Greek. This allophone has disappeared in Modern Greek.
 * PIE > Ancient Greek ῥέω "flow" — possibly