The Pennsylvania rule is a legal principle that applies to maritime law, specifically to cases of collision, which is when two moving vessels hit each other.

The rule states that there is a presumption of causation when a vessel violates a statutory rule of navigation intended to prevent collisions.

The rule places the burden of proof on the violating vessel to show that its violation could not have been one of the causes of the collision.

The Pennsylvania rule was established by the US Supreme Court in 1874, in the case of The Pennsylvania v. The Troop. The case involved a collision between two steamships in a foggy night near New Jersey. The Pennsylvania was found to have violated the law by ringing a bell instead of sounding a foghorn. The court held that this violation created a presumption that the Pennsylvania was at fault for the collision, unless it could prove that its violation could not have contributed to the accident.

The Pennsylvania rule is different from the Oregon rule, which is another legal principle that applies to maritime law, specifically to cases of allision, which is when a moving vessel strikes a stationary object. The Oregon rule states that there is a presumption that the moving vessel is at fault for the allision, unless it can prove otherwise. The Oregon rule was also established by the US Supreme Court in 1874, in the case of The Pennsylvania v. The Troop.

The Pennsylvania rule and the Oregon rule are sometimes used together in cases where both collision and allision occur, or where there is a dispute over whether the object struck by the moving vessel was stationary or not. In such cases, the courts have to weigh the evidence and arguments presented by both parties and decide which rule applies and how to allocate fault and liability.

The Pennsylvania rule and the Oregon rule are important for enforcing obedience to the rules of navigation and promoting safety at sea. However, they are also criticized as being too harsh and unfair to the violating vessels, as they impose a heavy burden of proof on them and may result in disproportionate liability.