A passing tone is a
non-chord tone used to fill in the gap between two other tones. These other tones can belong to the same
harmony, a different harmony, or may perhaps be non-chord tones as well.
Usually the space between the tones the passing tone separates is a some type of 3rd and the passing
tone is given the interval which lies between; this rule is not without
exception and especially during
modulations one may find a passing tone which seperates two tones of no less than a second apart.