Tunings

Although it is possible to play some slide guitar in the standard tuning (and this is a useful skill), slide guitar is best played in open tunings. Simply put, an open tuning is when your guitar is tuned so that strumming all the strings without holding any of them down on the fretboard will produce a chord. Open D (also known as 'Vastopol') and open G (also known as 'Spanish') are the most commonly used tunings for blues playing. These tunings have two advantages over other open tunings. Firstly, if a capo (a clamp used to hold all the strings down at one fret-position on the neck) is placed on the neck of the guitar at the second fret, the tuning is changed to E if using the D tuning and A if using the G tuning, which are very common keys for blues music. Secondly, both of these tunings have the string tension lower than in standard tuning, making it easier to fret notes with your fingers. This is a big advantage when playing an instrument which, being suitable for slide playing, has an action higher than normal. These are the two tunings we will primarily be working on.

I have included the open E tuning as it is an excellent tuning for electric guitar. Open E, as mentioned before, can also be achieved by tuning to open D and then placing a capo on the second fret. You will notice that the open G and open D tunings are "down" tunings, that is to say, they are achieved by de-tuning some of the strings from the standard tuning. The open E tuning is an "up" tuning, achieved by increasing the tension on three of the strings. Many players prefer not to use "up" tunings on acoustic instruments as they place extra strain on the sound-board and neck of the guitar. The "Fake" open G is a versatile tuning. You will notice, if you compare it to the standard tuning, that although the 1st to 4th strings are tuned to the same notes as open G, the 2nd to 6th strings are also the same as in the standard tuning. This makes it possible to play "open" slide in this tuning while still being able to play standard chords.
A lot of players say they don't like playing in open tunings because it's "a hassle" to re-tune their instrument. However, once you are familiar with these tunings it becomes very easy to switch between them. Keeping an electronic guitar tuner handy helps too!