Phonological history of English open back vowels

The phonology of the open back vowels of the English language has undergone changes both overall and with regional variations, through Old and Middle English to the present. The sounds heard in modern English were significantly influenced by the Great Vowel Shift, as well as more recent developments in some dialects such as the cot–caught merger.

Overview

Old and Middle English

In the Old English vowel system, the vowels in the open back area were unrounded: . There were also rounded back vowels of mid-height: . The corresponding spellings were and , with the length distinctions not normally marked; in modern editions of Old English texts, the long vowels are often written , .

As the Old English (OE) system developed into that of Middle English (ME), the OE short vowel merged with the fronted to become a more central ME . Meanwhile, the OE long vowel was rounded and raised to ME . OE short remained relatively unchanged, becoming a short ME vowel regarded as , while OE long became ME (a higher vowel than ). Alternative developments were also possible; see English historical vowel correspondences for details.

Later, ME open syllable lengthening caused the short vowel to be normally changed to in open syllables. Remaining instances of the short vowel also tended to become lower. Hence in Late Middle English (around 1400) the following open back vowels were present, distinguished by length:

• , spelt , as in dog, god
• , often spelt , or before consonant+vowel or certain consonant pairs, as in boat, whole, old

16th-century changes

By 1600, the following changes had occurred:

• The long vowel of boat had been raised to as a result of the Great Vowel Shift.
• The diphthong found in words such as cause, law, all, salt, psalm, half, change, chamber, dance had become an open back monophthong or .
• At this time, the short in dog was lowered to

There were thus two open back monophthongs:

• as in lot
• or as in cause

and one open back diphthong:

• as in low

17th-century changes

By 1700, the following further developments had taken place:

• The diphthong of soul was raised to , and then monophthongized to , merging with boat (see toe–tow merger). Before , this vowel further merged with due to the horse–hoarse merger except in some varieties, as currently seen in Irish English, Scottish English and African American Vernacular English.
• Short was retracted and rounded to . The shift was suppressed before a velar consonant, as in quack, twang, wag, wax, and also was suppressed by analogy in swam (the ("strong-verb") past tense of swim). The change of to did not occur in Mid-Ulster English.
• had begun to partake in lengthening and raising before a nonprevocalic voiceless fricative or /r/. That resulted in words like broth, cost and off having instead of , and was the start of the split (see further below).
• In words such as change and chamber, the pronunciation was gradually replaced in the standard language by a variant with , derived from Middle English . That explains the contemporary pronunciation of these words with .
• However, when preceded , as in laugh and half, was shifted to instead, derived from Middle English .
• An unrounded back vowel developed, found in certain classes of words that had previously had , like start, father and palm.

That left the standard form of the language with four open back vowels:

• in lot and want.
• in cloth and cost.
• in start, father and palm.
• in tor, cause, and corn.

Later changes

From the 18th century on, the following changes have occurred:

• The three-way distinction between , , and was simplified in one of two ways:
• In General American and old-fashioned RP, was raised to , merging with the vowel in (the cloth-thought merger).
• In many accents of England, the lengthening of the set was undone, restoring the short pronunciation . This became standard RP by the mid-20th century.
• In General American, the lot vowel has become unrounded and merged into (the father–bother merger).

This leaves RP with three back vowels:

• in lot, want, cloth, and cost.
• in tor, cause, and corn.
• in start, father, and palm.

and General American with two:

• in lot, want, start, father, and palm.
• in tor, cause, corn, cloth and cost.

Variation in present-day English

Unrounded

In a few varieties of English, the vowel in lot is unrounded, pronounced toward []. This is found in the following dialects:

• Most of Irish English
• Much of the Caribbean
• Norwich
• The West Country and the West Midlands of England
• Most of North American English
• Excluding northeastern New England English and Western Pennsylvania English, where it is typically raised toward , merging with the vowel in thought.

There's also evidence for it in South East England as early as the late 16th century and as late as the 19th century.

Linguists disagree as to whether the unrounding of the lot vowel occurred independently in North America (probably occurring around the end of the 17th century) or was imported from certain types of speech current in Britain at that time.

In such accents outside of North America, lot typically is pronounced as , therefore being kept distinct from the vowel in palm, pronounced or . However, the major exception to this is North American English, where the vowel is lengthened to merge with the vowel in palm, as described below. This merger is called the merger or more commonly the father–bother merger. (See further below.)

Father–bother merger

The father–bother merger is a phonemic merger of the lexical sets and . It represents unrounded lot, as detailed above, taken a step further. On top of being unrounded, the length distinction between the vowel in lot and bother and the vowel in palm and father is lost, so that the two groups merge. This causes father and bother to become rhymes.

This occurs in the great majority of North American accents; of the North American dialects that have unrounded lot, the only notable exception to the merger is New York City English, where the opposition with the -type vowel is somewhat tenuous.

Examples of possible homophones resulting from the merger include Khan and con () as well as Saab and sob ().

While the accents in northeastern New England, such as the Boston accent, also remain unmerged among older speakers, lot remains rounded and merges instead with cloth and thought.