the foundation at the base of the door (see Molina; transl. here to English by SW)
Orthographic Variants:
tlatzacuillotl ytzintlan
Alonso de Molina:
tlatzacuillotl ytzintlan. el quicio baxo dela puerta. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 142v. col. 1. Thanks to Joe Campbell for providing the transcription.