Once you have mastered some triads (chords that have no numbers in their name), it is time to widen your knowledge and learn a couple of seventh chords. A seventh chord is made up of a triad plus one extra third. It means that if for example in C-major you have three notes: c-e-g, then by adding one more third you would get a seventh chord. Wait a minute – but if you count seven steps up from C, then you get B? Yes, and in this case you get a major chord with major seventh. The name is then Cmaj7. But if you have to play C7 instead, then you’ll have to lower the B so it becomes B-flat. And the most common chord shape is this:
You can see the fingering from here:
Now go and practice this chord with automated feedback at strumprofessor.com to make sure it sounds right. If you cannot wait until the C7 comes, you can build a custom level where you put exactly the chords that you want to practice.