Chord extensions are additional notes added to a basic chord, which typically consists of three notes: the root, third, and fifth. By including extra notes, such as the seventh, ninth, eleventh, or thirteenth, musicians can create richer and more complex sounds. These extensions enhance the harmonic texture and can evoke different emotions in music. For example, a C major chord consists of the notes C, E, and G. When you add a B (the seventh), it becomes a C major 7 chord. Similarly, adding a D (the ninth) transforms it into a C major 9 chord, further expanding its harmonic possibilities.