Grammar: calibre or caliber?
The British spelling is 'calibre'; 'caliber' is American English.
(On an interesting aside, since the launch of the app Calibre, many Americans have been asking themselves: 'How do I pronounce Calibre'. The answer is: the same way that you say caliber. Just because the US spelling is -er and the British -re, there is no difference in sound. Despite that, many Americans call the program "cal-ee-bray", which is wrong. It is always "cali-ber".)
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.