In a variety of Sasak called Ampenan Sasak in this paper, traditional documentation and analytical methods based on auditory perception reveal allophonic patterns in alternations of height among mid-vowels. High mid-vowels occur in final syllables ending in [ʔ] or no-coda (e.g. [tokoʔ] ‘fish species native to Lombok’) while low mid-vowels occur in final syllables ending in all other consonants (e.g. [tɔkɔl] ‘to sit’). However, words deviate from these patterns in several minimal pairs (e.g. [bəɾəmbok] ‘to discuss’ and [bəɾəmbɔk] ‘to breathe’) and in some borrowings (e.g. [agostos] ‘August’), suggesting a quasi-phonemic status for back mid-vowels; they behave like both phonemes and allophones. This study analyzes the phonetic properties of mid-vowels through an acoustic analysis of the F1 and F2 of 2,448 vowel tokens. Results suggest that mid-vowels are largely predictable among non-borrowed vocabulary. In final syllables, syllable openness serves as a predictor for the height of mid-vowels. In pre-final syllables, syllable openness has no effect on the height of the vowel. Rather, the height of pre-final mid-vowels is predictable based on the height of the final-syllable vowel. In consideration of both elicitation and acoustic evidence, this paper adopts a descriptive approach by stating that Ampenan Sasak back mid-vowels are largely predictable, with some exceptions. Further, the paper questions whether all sounds must be identified as a ‘phoneme’ or an ‘allophone’ and argues that quasi-phonemic segments are a valuable intermediate descriptor for both phonological theory and language documentation.