This paper provides a unified phonologically motivated explanation for the movement of trap, dress, and kit following the low-back merger in North American English (i.e., the Canadian Shift, California Shift, Low Back Merger Shift, Third Shift, etc.). The explanation puts forth that the three-way merger of lot, palm, and thought results in the loss of the [+Front] feature specification for trap, opening the door for dispersion focalization to pull trap toward the low central region of the vowel space. Analogy then prompts all other [−Peripheral] vowels, including strut and foot, to centralize. Crucial to this explanation is that the low-back merger includes palm, not just lot and thought. Evidence for this requirement is presented in a phonetic analysis of older speakers from conservative Victoria, British Columbia. The explanation presented here reconciles an earlier proposal (Roeder & Gardner, 2013) with Fruehwald's (2017) observation that parallel movement requires a shared feature specification.

This study provides the first wide-scale, apparent time, instrumental description of the Canadian Shift in mainstream Toronto English. In contrast with some previous findings, the Toronto data suggest that for the last 70 years or more the shift has not affected the high front lax vowel (i). We observe that the movement of the nonhigh front lax vowels (ε) and (æ) involves both lowering and retraction in Toronto English, although retraction is the primary direction of more recent change and the shift appears to be slowing down. Our findings also suggest that continued retraction of the vowel resulting from the low back merger is involved in the final stage of the shift. We do not find evidence of a chain shift but instead propose that a parallel shift is occurring and make reference to Vowel Dispersion Theory in our discussion.