Grammar: well-known or well known?
well known: So why do we write about a well known person and not a well-known person? You would in many ways be justified in wanting to write about a well-known person because you would also write about a denim-clad cowboy, a hearing-impaired pensioner and a straight-laced accountant. Here, the words joined by the hyphen make up a compound adjective (making one, precise description by using two words – and joining them). The reason why we don't use a hyphen with well and known is because – unlike denim, straight, impaired etc – well is an adverb (check the dictionary): it gets treated like any -ly word and does not need the hyphen.
So adverbs are used just like a highly skilled professional, a seasonally adjusted schedule and a fully understood piece of advice – so too, a well thought-out plan, a well known person and a well worn pair of jeans. Now, taking all this into account, why do we keep seeing well-known all over the place. The reason is, just as I mentioned above, that to a lot of people the hyphen between the well and known simply looks more logical than without. Even the Times (UK) advises in its style guide to hyphenate well and another word simply because the "compound looks better" with a hyphen than without. They suggest not using the term altogether. They also point out that ill, as in ill equipped, is also an adverb and does not technically need a hyphen – but you try convincing people of that! Luckily, too, being well known (n.) is also spelt without a hyphen.
About the author
Jesse Karjalainen is the author of The Joy of English: 100 conversations about the English Language, Cannibal – the language and history of the discovery of the New World, and Roanoke – the language and history of Early Colonial America.