IM JK

In the olden days, before the mouse was invented, players used IJKM to navigate in their PC games.  It was modeled after the joystics of arcade games, positioned on the right.  At that time, most games were third-person and required the player to move in any direction rapidly.  Therefore, a finger was used for each direction.  This is why they chose IJKM and not IJKL.  The more you know.