Q: My son recently asked me why “ough” words (like “ought,” “tough,” and “though”) are pronounced so differently. Can you help?
A: The combination of “gh” after vowels (or vowel pairs) has given English a lot of odd-looking spellings, including those of the “ough” words your son has asked about.
The short answer is that these peculiar spellings represent people’s attempts in the past to write words the way they sounded. Eventually spellings were standardized, but pronunciations kept evolving. This is why many words don’t look the way they sound.
We’ve written about this occasionally on our blog. In a 2009 post, for example, we note that “daughter” and “laughter” once rhymed.
In fact, “daughter” has had several pronunciations over the centuries, including DOCH-ter (with the first syllable like the Scottish “loch”), DAFF-ter (rhyming with “laughter”), and DAW-ter.
The Middle English letter combination “gh” is now pronounced like “f” (as in “cough,” “trough,” “laugh,” “enough”) or not at all (“slaughter,” “daughter,” “ought,” “through”).
As we say in the earlier post, “Much of our modern spelling had its foundation in the Middle English period (roughly 1100 to 1500). But in the late Middle English and early Modern English period (roughly 1350 to 1550), the pronunciation of vowels underwent a vast upheaval.”
Linguists call this the Great Vowel Shift. While the pronunciations of many words changed dramatically, their spellings remained largely the same. That’s because printing, which was introduced into England in the late 1400s, helped retain and standardize those older spellings.
As for those “ough” words, Robert Heinlein strings six of them together in The Door Into Summer (1956). In the novel, an inventor comments on the “illogicalities” of “a language in which you could say: ‘Though the tough cough and hiccough plough him through.’ ”
from Grammarphobia http://www.grammarphobia.com/blog/2017/05/ough.html