Fatto generale: 84 is the atomic number of polonium.
Fatto matematico: 84 is also the lim sup of the largest finite subgroup of the mapping class group of a genus g surface divided by g.
Trivia: 84 is the code for international direct dial phone calls to Vietnam.
Data storica: March 24th is the day in 1934 that U.S. Congress passes the Tydings-McDuffie Act allowing the Philippines to become a self-governing commonwealth.