We have two things to explore: Open Positions and Open Keys.
An Open Key is any key which contains notes found on open strings. So any Key which contains an E,A,D,G or B note would be an open key. Not All keys are open keys, there are only 11:
C major/A minor, F major/Dminor, Bb Major/G minor, Eb major/C minor, Ab Major/F minor, G major/E minor, D major/B minor, A major/F# minor, E major/ C# minor, B Major/ G# minor, F# major/D# minor.
Obviously you can play these keys all over the neck. However, when you play them at the lowest possile position we will refer to this as the Open Position.
Here are the 11 open keys in open position.
Try the following exercises:
1)Play the scales ascending and descending
2)Play the Major Chord and then practice the scale
3)Play the minor Chord and then practice the scale.
4)Practice holding a chord and playing melodies using notes from the scale with free fingers
5)Practice playing a chord progression and filling between the changes with riffs built off the scale.
If you’re already an advanced player you can use these positions to play over more comlplex changes. EG Take the A train, Rhythm Changes, Giant Steps, Sing a song of songs, etc.