Parallel fourths
Two voices moving in parallel a fourth apart; judged acoustically unfavorable by older theory.
What it means
Parallel fourths arise when two voices move in the same direction at the distance of a perfect fourth, so that the interval of a fourth is retained. Older theory judged them unfavorable in sound, above all between the lowest voice and another, because the fourth above the bass counted as a dissonance. They give no independence to the voices but make them stick together.
Musical use
The prohibition belongs to the rules of voice-leading in strict writing, similar to the prohibition of parallel fifths and octaves, but is less severe. In medieval organum parallel fourths were on the contrary used deliberately, and in the music of Impressionism, for instance in Claude Debussy, as well as in jazz with its quartal harmony, they are an intended tonal device. The judgement therefore depends on the style.
Parallel fourths should be distinguished from parallel fifths and octaves, which count as a serious error in strict teaching, and from parallel thirds and sixths, which count as pleasing.
Performance practice
When you write a texture, check parallel fourths critically: in strict exercises they are to be avoided. As a color in modern music you may use them deliberately and note their open, hollow effect.