The story
The American Association of Physics Teachers was founded on December 31, 1930, when forty-five physicists held a meeting during the joint APS-AAAS meeting in Cleveland specifically for that purpose.
The AAPT became a founding member of the American Institute of Physics after the other founding members were convinced of the stability of the AAPT itself after a new constitution for the AAPT was agreed upon.
From Wikipedia’s article on American Association of Physics Teachers, shared under CC BY-SA 4.0.3