What's the Difference Between 'Historic' and 'Historical'?
And should you use 'a' or 'an' before each one?
Historic and historical: what a pair. First off, there's the question of whether to use a or an before them. Both a and an are used, but a is far more common—as much as four times more common in American English, by some measurements—which is what you'd expect for a word that, like habit and hero, begins with an audible \h\.
So why do some people say "an historic" and "an historical"? Well, historic and historical differ from habit and hero in a crucial way. They have their accented, or stressed, syllable second, not first. It used to be that an initial "h" was not pronounced in many such words, which is how "an historic" and "an historical" came to be used in the first place. Both words are now, however, typically pronounced with an audible \h\. (A vs. an can be tricky: in fact, we've written an entire article about it.)
As for which word to use where, the answer is similarly complicated.
The two words are at their core simply variants, but over 400-plus years of use, they've mostly settled (emphasis on "mostly") into distinct roles. Historical is the typical choice for the broad and general uses relating to history. It's the one used to modify words like museum and society, and it's the one found in contexts like these:
… how much did Shakespeare's "Henry V" have to do with the historical monarch who fought the battle of Agincourt?
— Andrew O'Hehir, Salon, 7 Oct. 2015
George Washington, who has been cited as the first American to have made historical note of the avocado, wrote of encountering "agovago pairs" on a trip to Barbados in 1751.
— Andrea Nguyen, Saveur, August/September, 2007
Historic is most commonly used for something famous or important in history:
It has been billed the most historic and oldest regatta in the county….
— Sue DeWerff, Florida Today, 16 May 2013
Nestled in the country's southwest border alongside Switzerland and France, Baden stretches from Lake Constance's glistening shores along the brooding Black Forest to historic Heidelberg.
— Anne Krebiehl, Wine Enthusiast, February 2014
People who write about matters such as these tend to pretend that the differentiation is more absolute than it is; there are, in fact, instances to be found in which skilled writers apply one word where the other is typically found, and vice versa. Still, the distinction outlined above is a good one to follow if you want to communicate efficiently with your reader.