songs to learn slide playing with

Open C is great if you like a really lowdown sound, it's the one I use most, aside from open D, and open Em,

....Open C is C G C G C E

Open C's a cool tuning, all right! Not one of the traditional blues tunings, it's pretty popular with fingerstylers. What's neat about it is that it's a third arrangement of the open major chord tunings. Open D and its analogues have the third string tuned to the third. Open G et al have the second string tuned to the third. Open C has the first string tuned to the third. There are things that'll sound cooler or be easier to play in one or the other.
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