The rhotic or "r-like sound" of Standard European French is a segment varying in manner, both degree of aperture and voice quality. This paper presents a broadly functional analysis of // voicing within an Optimality Theoretic framework, accounting for instances of voice assimilation, neutralization and apparent free variation. Autosegmental analysis of // voice alternation is critiqued based on its inability to account for voice patterns. A lenition-based approach, taking into account active and passive voicing mechanisms, is proposed, from which alternations between voiced and voiceless // are seen as the result of principled phonological processes involving effort avoidance and reduction. This analysis supports a representation of // that is unspecified for active glottal control; essentially, the output or surface instantiation of // derives from its phonological environment, as constrained by universal principles of effort reduction or avoidance. These principles are articulated in grounded constraints LAZY(glot), targeting glottal or laryngeal effort. Other issues raised include the integration of phonetic principles in phonological explanation, the justification for input or underlying representations of variant phonemes and the heuristic capabilities of proposed constraints. A concluding section comprises a discussion of the data and the conception of voice as a phonological category.