-A songs key signature will determine the scale that should be used, f.ex if the song is in A major, there's going to be a lot of the A major scale being used in the songs in its riffs, licks and solos.
-You can use the "octave triangle" to locate the octaves in a certain key. Octaves work differently on the guitar then the piano.
-Overtones. They seem most common for vocals, piano and violin. They might be used for guitar in some fashion, however i'm not sure.
-Pentatonic scales consists of just 5 notes, hence "penta". Major and minor penatonic scales share the exact same notes. Penatonic scales are simplified versions of major and minor scales. Comparing the major and major penatonic scale, the penatonic scale admits the 4th and 7th degrees. Songs that are 100% melodically and harmoically within the pentatonic scale are very rare in western music.
