Nikolai Rimsky-Korsakov · section 6 of 96
Seventh Chords
Read and hear this section in the playable edition →§ 3. A seventh chord consists of a triad with a further third added, which forms a seventh above the bass:

It can also be regarded as made of two triads:

Seventh chords are classified as follows:

All seventh chords are figured 7.
§ 4. Every seventh chord has three inversions: 1st, the chord of the fifth and sixth (6/5); 2nd, the chord of the third and fourth (4/3); 3rd, the chord of the second (2).

The inversions are told apart by where the second lies — the interval that comes from inverting the seventh: at the top in the first inversion, in the middle in the second, and at the bass in the third.
An inversion keeps its name whatever the arrangement of the three upper parts. For example, the following chords

are always regarded as chords of the fifth and sixth, because the third of the root is in the bass.
The chords

are always regarded as chords of the third and fourth, because the fifth of the root is in the bass.
The chords

are always regarded as chords of the second, because the seventh of the root is in the bass.