The restricted area arc and the charge it prevents
Court markings
What the small semicircle under the basket is for, and how it changes a block-or-charge decision in real time.
Under each basket there is a semicircle with a four-foot radius, drawn from the centre of the rim. It is the restricted area, and it exists to answer one argument that used to happen several times a night: whether a defender standing directly under the hoop can legally take a charge from a player driving at full speed.
The rule is simple. If a defender has both feet inside that arc and is not actively trying to block the shot in a vertical position, he cannot draw an offensive foul from a driving player. Contact in that spot is a blocking foul on the defender. Outside the arc, the normal rules apply: if he got there first, set his feet, and gave the attacker room to change direction, it is a charge and the offence loses the ball.
The arc exists for safety as much as fairness. A player leaving the floor from six feet away has no way to stop, and a body appearing underneath him is how ankles and backs get hurt. Drawing the line gave everyone a shared, visible threshold instead of a judgement call.
There are exceptions worth knowing. The restricted area does not protect a driver against a secondary defender who arrives legally and jumps straight up, because that is verticality, not a charge attempt. It also does not apply to rebounding contact or to post play, where nobody is airborne in the same way. And a defender with one foot in and one foot out is treated as inside.
Watch what good defenders do with it. Instead of planting under the rim, they position one step outside the arc, usually just off the driver's shoulder, and wait. From there a charge is still available. The offence knows this too, which is why so many drives finish with a step toward the baseline: the attacker is trying to get past the spot where the defender is allowed to stand still.
If you are watching a replay of a collision near the rim, look for the paint on the floor before you look at the players. Where the defender's heels were relative to that semicircle usually tells you the call before the referee signals it.