Which sound change affected English long vowels from the 15th to 18th centuries?

The sound change that affected English long vowels from roughly the 15th to 18th centuries was the Great Vowel Shift.

It was a chain of pronunciation changes that transformed the long vowels of late Middle English, mainly in southern England. The shift began around the 1400s and continued through the 16th and 17th centuries, with some related developments extending later. Close vowels such as the one in Middle English “time” became diphthongs, while several other long vowels moved higher in the mouth.

The shift helps explain why English spelling often preserves older pronunciations. Words such as “bite,” “meet,” “mate,” and “boot” changed their vowel qualities while spelling conventions were becoming more fixed. As a result, English letters frequently represent sounds differently from their historical values.

The Great Vowel Shift was not a single uniform event affecting every dialect in exactly the same way. Its causes remain debated, with proposed influences including regional migration, dialect contact, and internal phonetic pressures. It also differed from Grimm’s law, an earlier set of consonant changes affecting Germanic languages.