If you were to choose any key, then chart out every note on a string instrument's neck; you would see 5 distinct hand positions.  In music, there are 12 possible pitches with 7 in a key and 5 out of key.  Any key of your choice is related to other modes (scale types): for instance, C Major scale uses the same notes as does A minor, D minor with a Major 6th in the scale, and G Major with a minor 7th in the scale.  The minor scale with a Maj 6th in it seems to be the scale mostly used as the blues scale; sometimes they use pure minor.  Many times a popular song in a Major key will use the Major with a min 7th.  These 5 positions overlap and always go in order up the neck.  For a visual of the fingering is the GUITAR SLIDE RULE - 5 POSITIONS on EBAY, also available is the BASS SLIDE RULE, FRETLESS BASS SLIDE RULE, the MANDOLIN SLIDE RULE, the BANJO SLIDE RULE, the VIOLIN SLIDE RULE, the VIOLA SLIDE RULE, the CELLO SLIDE RULE, the STRING BASS SLIDE RULE, the UKULELE SLIDE RULE and the PIANO SLIDE RULE on EBAY.   These slide rules are perfect for showing where all the notes are for playing a lead or adding extra fill in notes.  You pick a key, set the slide, and play notes and combinations of notes from the pictures in front of you and every note will be in that key.   Improvise!  Usually, the first chord of a song is the key to improvise.  Put on some familiar music of which you know the first chord and play along using different combinations of the notes showing on a slide rule.   All the notes showing will be in that key in 5 easy hand positions.  Here are a few slide rules available on eBay: