langue d'oïl
Definition - The version of Old French spoken in the northern half of France (and in parts of Belgium and Switzerland) that evolved into standard Modern French.
Note: The version of Old French that was spoken in Southern France was called langue d'oc.

Example -

Etymology -
Langue d'oïl was so called because its speakers used the word oïl — from the Latin hoc ille — for yes. This evolved into the Modern French oui. In contrast langue d'oc speakers used the word oc (which derives from the Latin hoc) for yes.

Oxford English Dictionary -
The term's first citation is from 1703:
"I more approve of the Etymology of those who observe, that, time out of mind, the French have been distinguish'd into Langue d'Ouy, and Langue d'Oc, that is, into such as say Ouy, and such as say Oc for Yes; the first living on this, and the other on that side the River Loire."
(Acct. Theatre of War in France, being a Geogr. & Hist. Descr. Languedoc 3,)

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