Bb

A good voicing for a 7th chord that forms the basis of an important moveable shape.

You can play this one open but it's not going to work well as a moveable shape. There's no 5 either

This is a useful shape because it's easy to grab and with the tonic so prominent there's no ambiguity when it comes to the chord changes but without the 3rd it lacks the fullness of a major or a minor chord. In this case it's in the 3rd position so it's a Bb chord.