In this article, we offer a historical account of the development of two phonemic ‘interior’ vowels, [ə] and [ɤ], and heterosyllabic vowel sequences in Ngwi, a virtually undescribed West-Coastal Bantu language spoken in the western part of the Democratic Republic of the Congo. While interior vowels phonologized due to the loss of their conditioning environment, most heterosyllabic vowel sequences come from the fission of an erstwhile palatal on-glide diphthong, itself originating in a long [–high] vowel. This origin is exactly the opposite of what has been reported for other language families such as Romance, where certain heterosyllabic vowel sequences of the *iV type evolved into diphthongs. The Ngwi data also show that phonemic interior vowels exist in languages of the Niger-Congo phylum where they had not been reported before. The historical development of heterosyllabic vowel sequences in Ngwi might have parallels in several other Bantu languages from the wider Congo rainforest area, where phonetic and phonological documentation is still embryonic.

Languages tend to license segmental contrasts where they are maximally perceptible, i.e. where more perceptual cues to the contrast are available. For strident fricatives, the most salient cues to the presence of voicing are low-frequency energy concentrations and fricative duration, as voiced fricatives are systematically shorter than voiceless ones. Cross-linguistically, the voicing contrast is more frequently realized word-initially than word-finally, as for obstruents. We investigate the phonetic underpinnings of this asymmetric behavior at the word edges, focusing on the availability of durational cues to the contrast in the two positions. To assess segmental duration, listeners rely on temporal markers, i.e. jumps in acoustic energy which demarcate segmental boundaries, thereby facilitating duration discrimination. We conducted an acoustic analysis of wordinitial and word-final strident fricatives in American English. We found that temporal markers are sharper at the left edge of word-initial fricatives than at the right edge of word-final fricatives, in terms of absolute value of the intensity slope, in the high-frequency region. These findings allow us to make predictions about the availability of durational cues to the voicing contrast in the two positions.