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.)
