The C major seventh is commonly known as the C7 guitar chord (pronounced C seventh). C7 uses the five highest pitched strings.
The chord diagram on the left is for right handed players, the diagram on the right is for left handed guitarists.
Play the C chord, then the C7 chord several times by pressing and removing your fourth finger. Remember to make it musical and have fun.
The C7 chord is also commonly used as a transition chord.
By now you will notice that the chords you've learned at the beginning of the chord lessons are becoming automatic to play! Practice the chord changes that are the most difficult for you, and soon they will become automatic too.
There's only one more basic guitar chord to learn now, the D7.