The basketball court has gone through numerous changes throughout the sport's history. There have been several sizing and dimension changes to the height and radius of the key and its semicircle. The first iteration of the key was much larger than what it looks like today.
In the modern game, players have chosen not to make as many shot attempts from the top of the key. The reason for this change is because shots from the top of the key are the most inefficient for players to take. Out of all two-pointer attempts it is the furthest spot away from the basket. Three pointers are only slightly further away from the basket and are worth three points instead of two.
Shots from the top of the key yield about 86 points per 100 possessions. That is far below the number for corner three pointers (119 points per 100 possessions). Due to that difference being uncovered teams have had their players take less shots from the top of the key in today's game.
The key or paint is the square box below each team's hoop. The key includes the free throw line. 3-second violations are called when a defensive player stays in the paint for longer than three seconds.
The NBA's key is 16 feet in diameter, the NCAA's is 12 feet, and FIBA's is also 16 feet but does not have the full circle.