The use of sources from this language in the 1952 thesis of Charles Gibson led James Lockhart to coin the field of New Philology. The Cundill Prize was awarded to a 2019 book based largely on neglected sources from this language by Camilla Townsend. Sources in this language chronicle the southward migration of speakers of this language like the Pipil and Nicarao peoples. Historians learned to write in this language at schools called calmecacs (10[1])(“kahl-MAY-kahks”). An early ethnographic work in this language at the Laurentian Library was written from questionnaires with students at the Colegio (“ko-LAY-hee-yo”) de Santa Cruz; that work by Bernardo de Sahagún (“sah-ah-GOON”), the Florentine Codex, depicts a native perspective on the conquest of Tlatelolco and Tenochtitlan. For 10 points, name this lingua franca of the Aztec Empire. ■END■ (10[1])