In Western music, harmony is the use of ifferent pitches simultaneously, and chords, actual or implied, in music. The study of harmony may often refer to the study of harmonic progressions, the movement from one pitch simultaneously to another, and the structural principles that govern such progressions. In Western Music, harmony often refers to the "vertical" aspects of music, distinguished from ideas of melodic line, or the "horizontal" aspect. For this reason, considerations of counterpoint or polyphony are often distinguished from those of harmony, though contrapuntal writing of the common practice period of western music is often conceived and defined in terms of underlying harmonic motion.