For 1990-2009 cars

The propeller shaft is designed and built with the yoke lugs (ears) in line with each other which produces the smoothest running shaft possible. A propeller shaft designed with built in yoke lugs in line is known as in-phase.

An out of phase propeller shaft often causes vibration. The propeller shaft generates vibration from speeding up and slowing down each time the universal joint goes around. The vibration is the same as a person snapping a rope and watching the wave reaction flow to the end. An in phase propeller shaft is similar to two persons snapping a rope at the same time and watching the waves meet and cancel each other out. A total cancellation of vibration produces a smooth flow of power in the drive line. Since phasing of a propeller shaft is between the front and center universal joints, you must reference mark the front and rear propeller shafts before removal in order to ensure proper phasing upon reinstallation. Some splined shaft slip yokes are keyed in order to ensure proper phasing.