McWhorter imagines that each language has four boxes that need to be checked if we, both as a culture and as individuals, are to fully develop our language skills. These boxes are formal speech, informal speech, formal writing and informal writing. We all, he argues, should "cultivate the four boxes."
By "formal speech," he doesn't mean in the sense of Edward Everett Horton and other great orators of the past. Except for a very few preachers, professors and politicians, nobody speaks like that anymore. Yet most of us do speak more formally in some situations than in others. We may speak one way on the job, especially when talking to customers or to our boss, than we do with friends. We may also speak differently to our mother, to members of the opposite sex, to teachers and to members of the clergy than we do in casual conversation. Most of us checked off those two boxes long ago.
Writing is another matter, however. That is something most of us learned to do in school, and our teachers taught us to write in a formal, or you might say proper, way. McWhorter, in one of his Great Courses lectures, points to letters written back home during the Civil War. Not everyone was literate back then, but those who were, even if they had no more than a few years of schooling, wrote in a flowery, very formal manner. That was how they had been taught to write. Today few of us write like that, or are even capable of writing like that, but even so writing tends to be formal in the sense of proper spelling and grammar. In writing, most people use complete sentences, say "going to" instead of "gonna" and so forth.
Until the advent of e-mail, texting and tweeting, however, there was no handy way to check off that fourth box. Virtually all writing, with the possible exceptions of diaries, telegrams and notes passed in classrooms, was formal writing. Now that fourth box has been checked.