In a seventh chord (strictly speaking dominant 7th chord or V7 chord) a seventh is added to a major triad. In a C7 this is a Bb and not B. In a maj7 chord a major 7th is added to the major chord In a Cmaj7 this would be a B and not Bb. More about maj7 chords here.
Instead of going up seven steps in the minor scale you can count back a whole step (or two half steps):
C → B → Bb
G → G7
To add a 7th to the "classic" G chord we take the root tone G on the high E string and lower it by two frets to get the seventh F.
Drag the slider under the chord shape to the right to change the G into a G7 with this method. Also check out the whole thing with displayed intervals and tones and watch carefully how the chord is changing.
G7 barre chord
To modify the G barre chord we can take the root tone on the D string and lower it by two frets to get the 7th F.
Additionally we can raise the fifth (5) on the B string by three frets (half steps) to get a seventh. We already explored that in the lesson about minor 7th chords.
How about E7 or F7 ?
Move that grip as a whole to get other dominant 7th chords with the root on the low E string.
C → C7
In the "classic" C major chord it is not possible to lower the C on the B String by two frets to get the 7th Bb. The only way is to raise the 5th (open G string) by three frets.
If our chord shape contains only one fifth we can raise it nevertheless. It's no problem to omit the fifth in a seventh chord (not for chords with b5 or #5). Root, third and seventh are essential for the character of a seventh chord.
C7 barre chord
We can also take the C major barre chord and lower the C on the G string by two frets.
Move that grip as a whole to get other dominant 7th chords with the root on the A string like e.g. A7 or Bb7.
D → D7
Change a D major into a D7 like this...
Also this grip can be moved like a barre chord to get other V7 chords with the root on the D string, e.g. a Eb7.
Why is a 7th chord called dominant 7th chord or V7 chord?
When we build up stacks of thirds over every note of a major scale (using only notes of the scale), we obtain the modal chords of the major scale. Depending on how much thirds we stack, we'll get triads or seventh chord (or even ninth chords, etc.).
A major chord with a seventh (not a major 7th) is only present on the 5th (V.) scale degree - the so called dominant. That's why this chord is called a dominant seventh chord or V7 chord.
However, in certain harmonical contexts we'll often find V7 chords on other scale degrees as well, without changing the main key.
Further chord shapes
There are quite a few different chord shapes for dominant 7th chords. The root does not always have to be the lowest note, for instance if you've got a second instrument (bass, piano,...) playing the bass note, or if you're packing more than one chord shape of the same chord into a bar to make your accompaniment more interesting.
This lesson is not intended for showing all possibilities. The construction of dominant seventh chords should be clear by now. The chord finder shows more V7 chord shapes - fiddling out others by yourself is of course strictly allowed!