A passing note, also called a passing tone, is a note, usually falling on a weak beat, that allows a smooth, scale-wise motion in tonal music by 'filling-in' the space between two primary notes. These primary notes are usually a third apart, with the passing note being the diatonic scale degree in between. However, other intervals may also have passing notes between them. Two or more passing notes may be used to smooth over a leap of a fourth; or a single, chromatic passing note may be used to strengthen the movement of a major second. Passing notes are among the most common and frequently used non-chord notes.