The second group of -ough words begins with a Middle English diphthong:
Middle English sound |
ME spelling |
ModE spelling |
ModE sound |
[ɔʊx] |
boughte
dough
cough |
bought |
[ɔ] |
dough |
[o] |
||
cough |
[ɔf] or |
In the case of bought and ought, the original Old English monophthong vowels had become, in Middle English, the diphthong [ɔʊ] under the influence of the fricative [x], which was subsequently lost. [x] is a voiceless velar fricative, heard for example in the Scots dialectal pronunciation of the word loch. Dough and though followed a similar path from Old English, though in these instances the Middle English diphthong later reverted to a monophthong and was lengthened; hence it became available for raising during the Tudor Vowel Shift.
With examples like cough and trough the fricative changed to [f], and the diphthong became a monophthong without lengthening.