From the archives...
A customer wrote us to ask, "I was wondering why the word One starts with an O but sounds like W."
This is a very good question! "One" is a very odd word. In terms of the phonograms and spelling rules, it is a true exception. About 2% of words contain a sound or spelling that doesn't follow the rules. Often it is the oldest and most common words in a language that are irregular, whether because a word's pronunciation shifted over time or because it retained the spelling from an earlier spelling system or language, and English is no exception to this.
With "one," it is the pronunciation rather than the spelling that seems to have shifted over time; its spelling actually makes a lot of sense. "One" is from the same root as Latin "unus" and German "ein" and is related to the English words "lone," "alone," and "only." We teach students about these related words and help them notice the shared morpheme when we introduce this word. In this way, we help them understand that although the spelling of the word is irregular, some sense of how it came about can be found when we look at the morphology.
The Oxford English Dictionary notes that the pronunciation with /w/ developed in the 15th century. Apple Dictionary adds some additional details about the history that I find fascinating: it states that the /w-/ developed before the 15th century and was occasionally represented in the spelling. It also states that this pronunciation was not accepted into standard English until the late 17th century.