The music is using partial chords with certain chord tones omitted.
It's common to omit the fifth of a chord, like the
G major, or the
Why can the fifth be omitted?
You can't omit it all the time. It depends on the chord type and very importantly the place of the root within the tonality.
Let's keep things simple and assume a key a
All chords on the various roots in
G major (
G-E) are major or minor chords except the chords built on the root at the seventh degree of the scale (
F#) which is a diminished chord.
The fifth of all those major and minor chords are perfect fifths whereas only the one diminished chord uses a diminished fifth.
From that info you can sort of say most chords in the key are assumed to have a perfect fifth. Only the single diminished chord needs to be confirmed by actually including the diminished fifth.
A more complicated view involves looking at chord inversions, voice leading, and common harmonic progressions in a key. That cannot be explained with just a few lines, but suffice to say that in the key of
G major the two tones
G can possibly be an incomplete
G major chord or
B minor chord. Adding the fifth would certainly clarify that. But, harmonic conventions work such that the two tones are most likely to be considered a partial
G chord and confirmed more by the voice leading and surrounding harmony than the presence of the chord's fifth.
Some rule of thumb type guidelines:
- major and minor chords need only the root and third
- diminished and augmented chords need their respective diminished or augmented fifths
- any seventh chord based on those major, minor, diminished and augmented triads requires the seventh
To get an understanding beyond rules of thumb, you probably should study harmony and voice leading.
...aren't seventh chords made up from 4 notes?
This seems like this should get a specific response. It gets into the picky detail of definitions.
A chord is pretty abstract. Chords only become clear in actual music after harmonic analysis. Sometimes the chords are obvious, other times not. Various chord tone omissions and displacements may occur.
A triad is a chord with three tones. In one sense that is just a theoretical concept. A
G major triad is the tones
G B D. But in actual music there may be omissions and/or doublings. In three part harmony you might have the full three tones
G B D, but frequently you could omit the
D and still have three parts where a tone is doubled like
G3 G4 B4 (the numbers are octaves to better show the doubling.)
A seventh chord is usually described as a four tone chord - a tetrad if you want to get a nerdy :-) - but another way to describe it is any chord with a proper chord tone a seventh above the root. In
G major the two tones
C could easily be considered an incomplete
D7 chord, because with have a
D root and a seventh above it. We don't absolutely need the third
F# and fifth
A, because the
C alone match only one seventh chord in
D7. If the
C move next to
B it especially confirms incomplete chords
When people use these various terms there are often assumptions based on context. Someone reading the tab you presented might speak of triads and seventh chords - or present chord symbols - without mentioning incomplete chords or tones omitted.