Multilingualism played an important role in the development of Standard English, but previous generations of scholars downplayed the multilingual element in its history to the extent of ignoring late medieval institutional code-switching altogether. In the late fourteenth and fifteenth centuries, code-switched record-keeping was routine practice, from prestigious institutions to private individuals, although no history of English mentions it and very little of it is published. In this chapter, I firstly establish my claim that multilingualism led to Standard English by showing that Standard English is the descendant of coalesced supralocal Englishes that adopted both the written conventions of Anglo-Norman and much of its content-word stock when code-switching practices were reversed. What this means is that instead of a Medieval Latin grammatical matrix containing Anglo-Norman and English words, fifteenth century scribes switched to an English grammatical matrix containing code-switched Anglo-Norman and English words. Standard English was the eventual outcome of this reversal. I then track the “monolingual origin” story, still repeated in textbooks today – namely, the story that Standard English supposedly developed mainly from the dialect of the “East Midlands”, or “Central Midlands”, or “Chancery English”, or a mixture of the above, depending on the textbook. I show that the “monolingual origin” story goes back to the early 1870s and consider the reasons for why the monolingual-source explanation prevailed for so long.