These two words are often used interchangeably, but I don't believe their meaning is identical. I think I generally know how to use them in context, but could someone explain what he difference is supposed to be?
closed as off-topic by Edwin Ashworth, user140086, Mari-Lou A, Rory Alsop, Rand al'Thor Jun 22 '16 at 14:44
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Undoubtably and undoubtedly do actually carry different meanings, but they are often, erroneously, used interchangeably:
Undoubtably and undoubtedly are both well-formed words with clear, distinct meanings, yet the former is often used in place of the latter, giving rise to the mistaken belief that undoubtably is always wrong. It’s not.
The distinction between them can be subtle; something that is undoubted is not doubted, while something that is undoubtable is not capable of being doubted. The two meanings often overlap, which is why mixing up the two words is often not a serious error, yet the difference is occasionally important.
For example, one might claim that in his or her opinion the Beatles are undoubtably the most influential rock band in history, yet we probably couldn’t say they are undoubtedly so, because even one who believes the Beatles to be the most influential band must acknowledge that there are rock critics who doubt this.
As you can see from Ngram undoubtably is quite rare compare to undoubtedly :
- "Undoubtably" is considered to be nonstandard English by some authorities (for example, Garner's Modern American Usage (2009)), and the term is seldom found in modern literary writing. The Oxford English Dictionary provides no examples of usage after 1513 and characterizes "undoubtably" as "? Obs.," wondering whether the term is obsolete.
- Nevertheless, many examples of its usage can be found in 20th- and 21st-century popular English and in contemporary academic journals.