In root position, the root, or note upon which the chord is based, is in the lowest position. From bottom to top, a C major triad in this position has the C in the bottom and the E and G above the C. It is denoted as I, indicating the scale degree on which it was built. If the chord is a seventh chord, and contains the 7th of the scale, B, in addition to C, E and G, then it is denoted as a I7 chord.
In first inversion, the third of the chord is in the bottom. In both the triad and the seventh chord, the E would be the lowest note of the chord, with C, G and B above it. For a triad, this is marked as a I6 chord, and in the seventh chord it is a I6/5.
In second inversion, the fifth of the chord is in the bottom. In our examples, G would be in the bass. The triad of G, E and C is marked as I6/4, and the seventh chord of G, E, C and B would be marked as I4/3.
Only chords with four or more notes can have inversions greater than second inversion. In third inversion, the seventh of the chord is in the lowest position. In our example, B would be the lowest note, with C, E and G above it. This is denoted as I2.
Melodies can be inverted as well. An inversion of a melody is the melody written upside down. Where the original melody goes up, the inversion goes down and vice versa. For example, if the original melody is A-D-C-D, the inversion would be A-E-F-E.