The sociolinguistic enterprise has demonstrated that speakers manipulate linguistic variants as they construct their speech style. Contrary to this expectation, this study introduces specific cases in which stylistic variation is highly constrained. We examine the verbal negative suffix in Kansai vernacular Japanese. We first demonstrate that this variable indexes speech style. We then show that in a few specific contexts, such as following the verb stem shir- ‘know’, speakers overwhelmingly use a single variant, in this case, shira-n ‘not know’. We point out that the unusual forms such as shira-n all have a high relative frequency compared to the other forms in their paradigms. Complex forms such as the English word insane, which occur more frequently than their parts (in+sane), are difficult to decompose. We claim that this phenomenon also impedes stylistic variation as forms such as shira-n are not readily decomposed into verb stem + negative suffix.