-ough words First Group

The first group shows two different developments from a long [u] sound:

Middle English sound	ME spelling	ModE spelling	MnE sound
[u:x]	bough plough ynough tough	bough plow	[aʊ]
[u:x]	bough plough ynough tough	enough tough	[ʌf]

[x] is the voiceless velar fricative as in Scots dialect loch.

The top examples show the loss of the final fricative, and the long vowel becomes a diphthong through the the Great Vowel Shift. When the fricative was retained and changed to [f], the vowel was shortened.