Generally, the propellers on both engines of most conventional twin-engined aircraft spin clockwise (as viewed from the pilot seat). Counter-rotating propellers generally spin clockwise on the left engine, and counter-clockwise on the right. The advantage of counter-rotating propellers is to balance out the effects of torque and p-factor, eliminating the problem of the critical engine.
In designing the Lockheed P-38, the decision was made to reverse the counter-rotation, making both engines critical, and having the "tops" of the propeller arcs rotating outwards, away from each other. Tests on the initial XP-38 prototype demonstrated greater accuracy in gunnery with the unusual configuration. The German World War II Henschel Hs 129 ground attack aircraft, Heinkel He 177 heavy bomber and Messerschmitt Me 323 transport's counter-rotating powerplants used the same rotational "sense" as the production P-38 did.
Counter-rotating propellers should not be confused with Contra-rotating propellers that share a common axis.
Some aircraft with counter-rotating propellers include: