We compare the complexity of idiosyncratic sound patterns involving American English /ɹ/ with the relative simplicity of clear/dark /l/-allophony patterns found in English and other languages. For /ɹ/, we report an ultrasound-based articulatory study of twenty-seven speakers of American English. Two speakers use only retroflex /ɹ/, sixteen use only bunched /ɹ/, and nine use both /ɹ/ types, with idiosyncratic allophonic distributions. These allophony patterns are covert, because the difference between bunched and retroflex /ɹ/ is not readily perceived by listeners. We compare this typology of /ɹ/-allophony patterns to clear/dark /l/-allophony patterns in seventeen languages. On the basis of the observed patterns, we show that individual-level /ɹ/ allophony and language-level /l/ allophony exhibit similar phonetic grounding, but that /ɹ/-allophony patterns are considerably more complex. The low complexity of language-level /l/-allophony patterns, which are more readily perceived by listeners, is argued to be the result of individual-level contact in the development of sound patterns. More generally, we argue that familiar phonological patterns (which are relatively simple and homogeneous within communities) may arise from individual-level articulatory patterns, which may be complex and speaker-specific, by a process of koineization. We conclude that two classic properties of phonological rules, phonetic naturalness and simplicity, arise from different sources.