This paper presents evidence for two claims: (a) that the underlying contrast between stops in Swiss German dialects is based purely on quantity and (b) that the duration of the stop closure is its sole reliable phonetic reflex, i.e. there is a geminate–singleton opposition acoustically manifested in long–short closure duration. Using production and perception data on initial, medial and final stops in Thurgovian, a dialect spoken in north-eastern Switzerland, we show that the pattern of phrase-medial contrast neutralisation supports both arguments: when the extra phonological length position of a geminate is not syllabifiable, the closure duration shortens and underlying geminates and singletons become indistinguishable. The perception data in particular make evident that closure duration is the crucial cue of the underlying contrast, because, in the absence of this phonetic correlate, listeners can no longer discriminate an underlying geminate from a singleton. The results bear not only on central issues concerning the representation of geminates but also on some intricacies of the phonology–phonetics interface.