Seventh chords are harmonies composed of four pitches. These pitches are the 1st, 3rd, 5th, and 7th of a given root. Seventh chords can exist in five different qualities: major 7th, dominant 7th, minor 7th, half diminished 7th and diminished 7th. In addition, the major and dominant 7th chords can be altered by augmenting the fifth. However, the most common qualities of seventh chords are major, dominant and minor. These are the chords that serve as structural components of most chord progressions. Other chords serve as transitional tools to move between these structural chords.
Major, dominant and minor seventh chords all possess a perfect fifth (in their unaltered forms). This causes the fifth to be a less important pitch when establishing the quality of the chord. The root determines the letter name of the chord. The quality of the third and seventh establish the chord as either a major 7th (major 3rd and major 7th), dominant seventh (major 3rd and minor 7th) or minor seventh (minor 3rd and minor 7th) chord. The fifth can be omitted from the voicing while still clearly establishing the quality of the chord.
This simple three note voicing of a seventh chord in which the fifth is omitted is called a shell voicing. The name is descriptive of the fact that the root, third and seventh represent the basic structural shell of the chord. When playing piano by yourself, you can use a shell voicing entirely in your left hand while playing the melody or a solo in your right hand. You can also split up a shell voicing by playing the root in your left hand while playing the third and fifth in your right hand. This technique is useful when accompanying another instrument or voice that is covering the melody.
When playing in a band, the bass player usually covers the roots of the chords in his or her playing. This frees the piano player to omit even the root from his or her shell voicings. Rootless shell voicings consist of just the third and the seventh.