This is for standard tuning. It will help you get your guitar back in tune if you do not have a tuner handy.
Most guitar players are familiar with the standard E-A-D-G-B-E tuning. Standard is the most common tuning used in most styles of guitar. The intervals between strings used in standard tuning can be kept the same but the pitch changed. Tuning each string down an equal number of steps allows the same chords and scale patterns to be used, but allows a lower key to be used. The same open chord pattern to play a song in A minor can be used to play the same song in G minor if the strings are all tuned down one step. A capo placed across the frets can raise the key in the same manner.
Dropped tunings lower the pitch of the sixth (thickest) string in relation to the other strings. Drop-D (D-A-D-G-B-E) is the most common form of dropped tuning. This tuning is most commonly used by rock, metal and classical guitar players. Drop-D tuning is commonly used to play songs in the key of D, using the open lower D string as a bass note for guitar parts. Some guitarists will also down tune the strings an equal number of steps from the Drop-D tuning to play in lower tunings.
An open tuning is any tuning that forms a chord without pressing on the fretboard. This allows a guitar player to strum all six strings of the guitar unfretted and produce a usable chord. Open tunings normally form the major or minor chord, but a more complex chord can be used. Open G (D-G-D-G-B-D) is the most common version of open tuning. Slide players frequently use this style of tuning because they need to form chords straight across a single fret due to the shape of the slide. Keith Richards of the Rolling Stones is well known for his use of Open G tuning. Keith Richards quite often uses chord changes that are physically impossible to perform in standard tuning. Open tuning allows the first chord to be formed with the index finger barring a fret (bar chording). This leaves the other three fingers free to make fast chord changes.
Instrumental tuning imitates the interval pattern of another type of stringed instrument. Using instrumental tunings will allow a guitar player to play parts reminiscent of those types of instruments a little bit easier. For example, a six-string violin tuning would consist of F-C-G-D-A-E. These tunings may be an octave above or below the original instrument. The open strings of a guitar in a violin type of open tuning would be lower in pitch than a violin because the guitar is a much larger instrument.
Regular Interval Tunings
The standard tuning of a guitar has the interval between most of the adjacent strings as a perfect fourth. The interval between the 3 (G) and 2 (B) strings is a major third. One method of alternate guitar tuning is to use a regular interval for all the strings. This can be any interval except for unison or octave, which would make all the strings the same note. A guitar with a perfect fourth regular interval tuning would be tuned to E-A-D-G-C-F.
Any combination of six notes can be a guitar tuning. There are tunings that do not fulfill the requirements of the other guitar tuning types. One example of this would be drone tunings, with all the strings on the same note (D-D-D-D-D-D). The Four and Twenty is another example, with the guitar tuned D-A-D-D-A-D. This tuning does not qualify as an open tuning because it only has two different notes. Open chord tuning requires the open strings to form a chord, which requires at least three different notes.