Glide cluster reduction

The pronunciation of the digraph ⟨wh⟩ in English has changed over time, and still varies today between different regions and accents. It is now most commonly pronounced /w/, the same as a plain initial ⟨w⟩, although some dialects, particularly those of Scotland, Ireland, and the Southern United States, retain the traditional pronunciation /hw/, generally realized as [ʍ], a voiceless "w" sound. The process by which the historical /hw/ has become /w/ in most modern varieties of English is called the wine–whine merger. It is also referred to as glide cluster reduction.

Before rounded vowels, a different reduction process took place in Middle English, as a result of which the ⟨wh⟩ in words like who and whom is now pronounced /h/. (A similar sound change occurred earlier in the word how.)

What is now English ⟨wh⟩ originated as the Proto-Indo-European consonant *kʷ (whose reflexes came to be written ⟨qu⟩ in Latin and the Romance languages). In the Germanic languages, in accordance with Grimm's Law, Indo-European voiceless stops became voiceless fricatives in most environments. Thus the labialized velar stop *kʷ initially became presumably a labialized velar fricative *xʷ in pre-Proto-Germanic, then probably becoming *[ʍ] – a voiceless labio-velar approximant – in Proto-Germanic proper. The sound was used in Gothic and represented by the symbol known as hwair; in Old English it was spelled as ⟨hw⟩. The spelling was changed to ⟨wh⟩ in Middle English, but the pronunciation remained [ʍ].

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