'In considering the use of grammar as a corrective of what are called 'ungrammatical' expressions, it must be borne in mind that the rules of grammar have no value except as statements of facts: whatever is in general use in a language is for that very reason grammatically correct. (Henry Sweet, 1891)
OED - Oxford English Dictionary
COCA - Corpus of Contemporary American English
BNC - British National Corpus
CGEL - The Cambridge Grammar of the English Language
CGEU - The Cambridge Guide to English Usage
CEEL - The Cambridge Encyclopedia of the English Language
