In music theory, contrapuntal motion is the general movement of two melodic lines with respect to each other. In traditional four-part harmony, it is important that lines maintain their independence, an effect which can be achieved by the judicious use of the four types of contrapuntal motion: parallel motion, similar motion, contrary motion, and oblique motion.
Parallel motion at an interval of a perfect fifth is known as parallel or consecutive fifths, and at an interval of an octave is known as parallel or consecutive octaves. These motions are generally avoided in traditional counterpoint because they offer the lines so little independence from each other.
Similar motion is motion in the same direction, but with the interval between them changing. In other words, both lines move up, or both lines move down, but the interval between them is different in the first chord and the second chord. For example Play (help·info):
Contrary motion is motion in opposite directions. That is, when one of the lines moves up, the other line moves down. If the voices always move by the same intervals (in opposite directions) they are said to be in strict contrary motion.
Contrary motion is important to maintain independence of melodic movement in contrapuntal writing.
Similar and oblique motion provide less independence than contrary motion, but more than parallel motion.