Just like building triads on each note in a scale turns out some neat results, seventh chords can also be built from a major scale.
Keep in mind that I'm going to be building these seventh chords exactly the same way as we built triads - from a major scale. All the notes in the chords below come right from that scale. The types of seventh chords we get are determined by the steps in the scale, just like triads were.
Let's review our triads and their types first (using a C major scale):
C E G - Major - I
D F A - Minor - ii
E G B - Minor - iii
F A C - Major - IV
G B D - Major - V
A C E - Minor - vi
B D F - Diminished - vii°
All we're going to do now is add an extra note to each triad:
C E G B - Major 7 - I∆7
D F A C - Minor 7 - iim7
E G B D - Minor 7 - iiim7
F A C E - Major 7 - IV∆7
G B D F - Dominant 7 - V7
A C E G - Minor 7 - vim7
B D F A - Half-Diminished 7 - viiØ7
Just like triads, this pattern will hold true in ANY major key! Practice picking a key and playing all the seventh chords in it. It should sound something like this:
Seventh Chords in a Major Key