The OPEN G SLIDE RULE CHART - GBDGBD is an 8.5"x 11" laminated chart showing every note in every key over the neck.  For instance, above each fretboard are 4 related keys.  The area on the end are the open strings.  If the first chord of a song is C Major, like the chords: C Am F and G, improvise using C Major fretboard or C Major/m7 fretboard.  Usually, the first chord indicates the key.  If the chords of a song begin in Em, use the fretboard for E minor or E minor/Maj6.  The idea is not a top to bottom chord, but a left and right movement to fill any riff or lead in any key.  Every note showing would be in that key along with your improvised leads or fill in riffs. Try it!