dominant 7th chords
When a jazz beginner starts to improvise over a II-V-I progression, he generally plays arpeggios corresponding to the chords of the sequence. In other words, he learns to play minor arpeggios over the minor chord (II), dominant 7th arpeggio over the 7th chord (V) and major arpeggio over the major 7 chord (I). This way the harmony is highlighted without taking risks. Then, comes the modes, the basic rules concerning modes is that to play dorian over the minor chord , mixolydian over the dominant and ionian over the major chord. In short, in the long-run there is nothing exciting. That's why there is a nice trick used by number of jazz players to add smoothness to a line over a dominant 7th chord. This is a little tip that makes all the difference, simply play the notes of a major 7th arpeggio over the V7 rooted on its b7 degree.