If you play the guitar you probably know how to make some type of Dominant 7th chord, but understanding exactly what “dominant” or “seventh” means may not yet be incorporated into your knowledge base. As an example, if we construct a G chord on the guitar, that places a finger on the third fret of the first string, the second fret of the fifth string, and the 3rd fret of the sixth string we have a very balanced G chord that would play most of notes of a G chord in two octaves.
The G chord has the notes of G, B, and D.. With the form of the G chord described the notes in order from the sixth string to the first string would be: G, B, D G, B, G. This would in effect be the root, third, and fifth of the G major scale and then the root, third, and root. Counting upward past D you would get to E and then to F# (for the G major scale). Because F# is seven tones above the G and because it is a note that is in the “major” scale use of it to extend a basic G major chord then makes it into a G major 7th chord.
Simply playing the F# at the second fret of the first string instead of the G at the third fret wound then create the set of notes: G, B, D, G, B, F#. This chord has four different notes, but does have the high G note lowered to F# of our original chord form. It is a strong sounding G major 7th chord because it has two G notes, and two B notes and strongly sounds the major 7th because it would be the last string hit in a typical strum of the strings.
If instead of the F# the F note is used at the first fret you would be using a “flat” seventh. This is usually called the “dominant” seventh. The reason it is called “dominant” is because the use of the flat seventh is much more common to use in music than the major 7th. The major seventh tone has a tendency to want to move “resolve” to the root tone. We have a “feeling” that the music should move toward the root name, which in the G major seventh would be back to G. The use of the dominant seventh creates a feel that the music could move upward or downward and relieves our desire to resolve the sound toward the root. The use of a dominant seventh then would then seem to allow more freedom.
The reason this is described as “seem to allow more freedom” is because musicians can learn to create much more interesting music through the use of extended chord forms that create interest by creating “resolution conflicts” and then creating resolutions to this in unique ways. Experiment with the G chord structure described as you modify it to play a G major 7th and then to the G dominant 7th chord. This is a good one to experiment with because you will be hitting the tones that are changing as the last part of the strum.
When you play songs that will be sung by a group of people that are not trained singers, the use of rather standard chords tends to help them follow the song structure. The use of chords like G major 7th tends to create difficulty because non-trained singers will want to move more toward standard chord structures while musicians using extended chord forms will be trying to move in non-traditional patterns to create interest.
As your musical skills build, try not to get “dominated” by dominant 7th chords, try major 7th chord forms and be creative with the use of both.