Why does the word “blood” have a pronunciation very different from other ones with double “O”, like moon, room, soon, etc.?

In the 14th century, “blood” and “moon” rhymed perfectly. Then a 300-year linguistic pileup shattered their pronunciations just as the printing press permanently froze their spelling.

Before this divergence, English words spelled with “oo” generally sounded identical. They shared a long “o” sound—pronounced like the modern “oh,” but held longer in the mouth. In Middle English, blood rhymed with the modern word boat. When the printing press arrived in England in 1476, early printers locked in this “oo” spelling to represent that specific long vowel.

Just as the spelling froze, spoken English entered a period of massive upheaval known as the Great Vowel Shift. Between 1400 and 1700, speakers began pronouncing their long vowels higher in the mouth. The long “o” sound in all of these words moved up to become an “oo” sound (/u:/). For a brief period, “moon,” “book,” and “blood” all rhymed with “food.”

Then their linguistic paths diverged. In the 16th and 17th centuries, a change called Early Modern English Shortening swept through the language. Vowels began to shorten when they appeared before certain consonants, particularly “d,” “t,” and “k.” Words ending in “n” or “m,” like “moon,” “soon,” and “room,” escaped this shortening entirely and kept their long /u:/ sound. But words like “book,” “foot,” “blood,” and “flood” underwent the shortening, their vowel relaxing into the shorter /ʊ/ sound heard in “put.”

If the changes had stopped there, “blood” would still rhyme with “good.” However, a final linguistic shift occurred in the 17th century called the FOOT-STRUT split. In southern England, the short /ʊ/ sound in many words dropped even lower in the mouth, unrounding into the /ʌ/ sound used today in “cut,” “mud,” and “strut.”

Because these changes spread haphazardly over decades, they caught different words at different stages. “Moon” stalled after the first shift. “Book” stopped after the second. “Blood” and “flood” went through all three shifts. Yet because 15th-century printers set their spellings in lead before the changes finished, they all still wear the exact same orthographic disguise.