Non-chordal notes are those that do not belong to the chord they are being played with or in. They usually sound discordant.
Yet a discordant note often blends in when it is played next to and in line with the two notes immediately before and after it. An example is note d played between c and e, like ade or edc, with the chord C. Note f sounds discordant in chord C played with the right hand in the key of C. The middle finger is not used much in the key of C.
Sharp and flat notes are usually played only as incidentals in the key of C. Yet chord B flat, with the note b flat in it, sounds nice in key of C.
