The present study attempts to characterize the aerodynamic conditions required for the production of apical trills and to account for some universal tendencies in the patterning of trills in terms of their aerodynamic and distinctiveness requirements. In order to ascertain the aerodynamic conditions required for trills, oropharyngeal pressure (P o ) and airflow were recorded simultaneously in two subjects producing voiced and voiceless trills. The backpressure during trills was intermittently bled with catheters of varying diameter, and thus impedance. It was found that (1) voiceless trills show a higher P o and a larger rate of flow than voiced trills, which generates friction noise across the lingual constriction; (2) voiceless trills are more robust to changing aerodynamic conditions but less distinct auditorily, as inferred from acoustic data; (3) the P o and airflow conditions for voiced trills and fricatives show very similar values, with trills showing a narrower range of allowable variation. The behavior of trills in varying aerodynamic conditions accounts for observed phonological patterns: the universal preference for voiced trills, the alternation between trills and fricatives, trill devoicing, and the lack of nasal trills. © 2002 Elsevier Science Ltd. All rights reserved.