This awesome little lick comes from the intro of 'Terraplane Blues' - one of Robert Johnsons' big hits selling 5000 copies when it was first released. Its a great song littered with lots of sexual innuendo and metaphor about his car not working properly anymore. I expect he used to play it with a slide in open A tuning E A E A C# E with a capo on the 2nd fret, but I'm playing it in standard with a capo on the 3rd fret. This is mainly because I suck at slide and altereed tunings!