From Ron Powers’ Mark Twain: A Life:
Twain became a stickler for grammar. Perfect grammar was “the fourth dimension,” he said, constantly sought but never found. (“I know grammar by ear only, not by note,” he confessed.) He was appalled by the subjunctive: “It brings all our writers to shame.” He valued brevity…. “An average English word is four letters and a half,” he observed, adding that he had shaved down his own vocabulary till the average was three and a half.