The analyzed data provide evidence for sound changes that involve cumulative effects in the phonologization of phonetic duration. The data come from Kashubian, an understudied, endangered language spoken in northwestern Poland, and illustrate two historical processes: preservation and loss of ultra-short vowels (jers) and compensatory lengthening. The unified analysis of the two processes hinges on a reinterpretation of phonetic vowel duration as phonological length. Phonetic duration is contextual: Vowels in head syllables, in open syllables, and before voiced consonants tend to be longer than vowels in non-head syllables, in closed syllables, and before voiceless consonants. The effects are cumulative in the sense that all three conditions must co-occur on a single vowel. The discussed changes provide support for phonological models that (i) allow phonological constraints to access fine-grained phonetic information and (ii) are capable of deriving cumulative effects. The data contribute to the typology of cumulative processes by providing novel evidence of alternations that are simultaneously conditioned by the prosodic and segmental context.