What does it mean for two voices to be in similar motion?

When two voices are described as being in similar motion, it means that both voices move in the same direction, whether ascending or descending. This concept is important in music theory, particularly in the context of counterpoint and harmony, as it influences how music is structured and how parts interact with each other.

In similar motion, the relative distance between the two voices can vary; they may remain close together or diverge more widely, but the key characteristic is that they are both moving in the same directional trajectory. This creates a sense of unity between the voices, as they follow a coordinated path—either both rising or both falling.

This concept contrasts sharply with other types of motion in music, such as contrary motion where the voices move in opposite directions, or static motion where one voice does not move while the other does. Understanding these different types of motion helps musicians and composers create more complex and interesting textures in their music, as they can intentionally choose to use similar, contrary, or static motion for specific expressive effects.