If one understands how the addition of the different accidentals to a natural interval affects its quality, it is relatively easy to spell or identify intervals that include accidentals. For example, adding a sharp to the upper note of a major second will increase the size of the second by a semitone, forming an augmented second. However, adding a flat to the upper note of a major second will decrease the size of the second by a semitone, creating a minor second. A double-flat on the upper note of a natural major second will decrease the size of the second by two semitones, thereby forming a diminished second.

Example 14-2 illustrates how the addition of an accidental to the upper note of a natural interval changes the quality of the interval. Click, or click and hold, the arrow buttonss to select a natural interval and then click an accidental for the upper note.

Example 14-2