Passing notes are non-chord notes which are approached and quit by step in the same direction. They can be either accented or more usually, unaccented. Unaccented passing notes are the most common type of melodic decoration in all styles of music.
Unaccented passing notes normally occur between two accented chord notes which are a 3rd apart. (“Accented” here means “falling ON the beat”).
In this undecorated example in four-part harmony, the soprano part moves by leaps of 3rd from F to A, to C and back to A. The alto part moves by a 3rd from C to E on beats 2-3. The bass moves by a 3rd from A to F on beats 1-2.
We can fill in these gaps by changing the note values to a quaver (8th note), and writing passing notes between the thirds:
Where the dominant note moves to the tonic note, two consecutive unaccented passing notes can be used. The interval between the two chord notes is a perfect 4th.
Here, two passing notes can be inserted between G and C:
In minor keys, avoid writing an augmented 2nd between the 6th and 7th degrees of the scale, because this interval sounds very harsh. Use the raised 6th degree of the melodic minor scale when adding the 6th as a passing note.
In this example, the third between A and C# (box A) can be filled with a passing note, but Bb (box B) creates a melodic interval of an augmented 2nd (Bb-C#) which is unadvisable. Use B natural (box C) instead.