The linguistic snobs over at the Oxford English Dictionary have accepted "tweet" — as in to post something on Twitter — into its exclusive language club because "it seems to be catching on" as a verb in its own right, which got us thinking: how did it get itself lower-case t status? Other brand name verbs don't work like that. Xerox the verb, for example is big X Xerox; the OED has Google the verb as capital G, too. Tweet, though, gets the little t — even in The New York Times per the very strict standards editor Philip B. Corbett (that is, if he allows the word at all). "When we do use it, it's lowercase, because it's not a trademark," he told The New York Observer last year. Most people, and now the OED, agree with Corbett's styling, but not exactly for the reason he cites.
The upper versus lower-case distinction does have to do with trademarks, just as Corbett suggested. "The simple answer is that 'tweet' isn't a trademark, or at least it didn't start as one," linguist Ben Zimmer told The Atlantic Wire. A word like Google, because it doubles as both the proper noun and verb — Google the company and Google "to search" — has always had an official trademark. And in that case, the verb version keeps the style of its proper noun brand-name.
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Unlike made-up nouns Google or Xerox, Twitter takes its name from a real verb. "Twitter is a 'suggestive name,' as it is based on an actual word, twitter, imitative of a bird chirping," Zimmer explained to the Wire. "And because of that suggestiveness, early adopter were encouraged to think of 'tweet' as a kindred term, since it too is an onomatopoetic term for a bird's chirping." Both tweet and Twitter as verbs remained acceptable for awhile. And while Twitter got the trademark from the get-go, tweet developed organically and only gained official US Patent and Trademark Office stamp of approval in 2011 — long after its colloquial usage began.
At the time of Coburn's controversial style memo, Twitter had already had the trademark. But because it developed as a verb on its own without association to any official branding it took the lower-case t. And, so, as it enters the very particular pages of the OED it will stay that way.