The First Heart Sound - Clinical Methods - NCBI Bookshelf - NIH

Think: "M1T1" The Mitral valve usually closes just a split second before the Tricuspid valve.

Physiologically, S1 is primarily caused by the : the mitral valve and the tricuspid valve . As the ventricles begin to contract, the pressure within them rises rapidly, forcing these valves shut to prevent blood from flowing backward into the atria. Anatomy and Physiology of S1