Voiceless alveolar trill

The voiceless alveolar trill differs from the voiced alveolar trill 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 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 that has disappeared in Modern Greek.
 * PIE > Ancient Greek ῥέω "flow", possibly

Features
Features of the voiceless alveolar trill:


 * Its place of articulation is dental, alveolar or post-alveolar, which means it is articulated behind upper front teeth, at the alveolar ridge or behind the alveolar ridge. It is most often apical, which means that it is pronounced with the tip of the tongue.

Occurrence

 * Alveolar

Voiceless alveolar fricative 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. It occurs allophonically in Czech.

Features
Features of the voiceless alveolar fricative trill:


 * Its manner of articulation is fricative trill, which means it is a non-sibilant fricative and a trill pronounced simultaneously.
 * Its place of articulation is laminal alveolar, which means it is articulated with the blade of the tongue at the alveolar ridge,