The Low-Back-Merger Shift is a chain shift of vowel sounds found in several accents of North American English, beginning in the last quarter of the 20th century and most significantly involving the low back merger (which collapses together the low-back vowel sounds: /ɑː/, /ɒ/, and /ɔː/ in words like PALM, LOT, and THOUGHT respectively) accompanied by the lowering and backing of each of the front-lax vowels: /æ/, /ɛ/, and /ɪ/ (in words like TRAP, DRESS, and KIT respectively).
The back and downward movement of all the front lax vowels was first noted as distinguishing certain California English speakers in 1987, and it was soon known by linguists as the California Vowel Shift. Then, it came to distinguish certain Canadian English speakers in a 1995 study, now known in that variety as the Canadian Shift; today, it helps define Standard Canadian English. The California and Canadian Shifts were initially reported as two separate phenomena, but the same basic pattern was next documented among some younger varieties of Western New England English, Western American English, Pacific Northwest English, and Midland American English, all in speakers born from the 1980s onward. Linguists have proposed possible relationships between the low back merger and the similarly structured shifts in these regional dialects, though no unifying hypothesis is dominantly agreed upon yet. Assuming the similar chain shifts found in Canada and various parts of the U.S. have a single common origin, a variety of names have been proposed for this trans-regional chain shift which, besides the low-back-merger shift, include the Third Dialect Shift, Elsewhere Shift, Short Front Vowel Shift, and North American Shift.
