The arc under the basket and what a defender cannot do inside it

Painted under each basket there is a semicircle, measured from the point on the floor directly below the centre of the ring, with a radius of 1.25 metres under international rules. It is usually a thin line and easy to miss on a wide camera angle, but it settles one of the most argued calls in the sport.

The rule it carries is simple. A defender who has both feet inside that semicircle cannot draw an offensive foul from a player driving to the basket. If the contact happens there, it is a blocking foul on the defender, and the driver gets free throws if he was shooting. Outside the line, with position established and feet set before the driver begins his upward motion, the same collision is a charge and the offence loses the ball.

The reason the marking exists is positional. Before it was introduced, a defender could simply stand under the rim, wait for a driver with no view of the floor below him, and take the contact. That is not defending the shot, it is harvesting a foul call, and it made attacking the basket unnecessarily dangerous. The arc pushed defenders back out to where they have to actually contest.

There are exceptions worth knowing, because you will see them. The protection does not apply to contact away from a drive: a defender inside the arc can still draw a foul from an attacker who lowers a shoulder, extends an off arm or leads with a forearm. It also does not apply to a rebounding battle or to post play with the ball back to the basket. It covers the specific situation of an attacking player moving toward the rim with the ball.

Watching for it is a good habit. When a collision happens under the basket and the officials go to the table to review, the arc is usually what they are checking, frame by frame, to see where the defender's feet were at the moment the driver left the floor. One heel on the line counts as inside. That single heel decides two free throws one way or a turnover the other, in a game that might be tied.

You can also see the arc shaping behaviour before any contact happens. Good rim protectors stand with their toes just outside it, a step further from the basket than instinct suggests, so that they keep the right to take a charge while still being close enough to challenge the shot with a vertical jump. The vertical part matters: a defender who jumps straight up and keeps his arms inside the frame of his own body is rarely penalised, wherever he is standing.