Strictly speaking, fretted instruments will be “Tempered” in that open strings are “Tuned” and the length of the strings may not be exactly equal. (Take a look at the bridge) These minor adjustments are made in order for the instrument to play well along the fret board. There may be slight variations between different guitars and this can be pleasant to the ear when guitars are played together.
Some guitars allow slight adjustment of the bridge position for each string, effectively altering the string length. Adjusting this length is sometimes called setting the intonation. Here are some common turnings.
Standard: E A D G B E – This is used by most guitarists
(12 string guitars are sometimes tuned down a tone D G C F A D especially if using a heavier gauge)
Drop D: D A D G B E – Lowers the sixth string from E to D. With your guitar in Drop D tuning you can form a power chord on the bottom 3 strings of any fret.
Double Drop D: D A D G B D – Which also drops the first string to D.
Drop C: C G C F A D – A full step down from Drop D with the same advantages.
Open G: D G D G B D – Sometimes called Spanish tuning, easy to play as with all open tunings.
Open C: C G C G C E – A popular 12 string guitar tuning occasionally used on 6 strings as well. The open strings give a deep C Major Chord.
Full Step Down: D G C F A D – Generally using slightly heavier strings for a deeper tone. This is also sometimes used for 12 string guitar tuning giving less stress on the neck.
Dsus4: D A D G A D (Dad Gad) – Used by some folk guitarists players. All the intervals are perfect 4ths except one very small interval of one whole step between the G and the A.
Open C Minor: – C G C G C Eb – This is a variation on Open C where the first string is flattened.
Open D Major: D A D F# A D – This tuning is very powerful and rich because of the voicing of the open strings. Also easy to play.
Open D Minor: D A D F A D – Open D Minor is a variation of Open D Major.
All fourths: E A D G C F – All strings tuned with a regular interval. Used by some Jazz guitarists amongst others. It simplifies major chords but can make variations, such as seventh chords, more difficult.
More discussion on this when I get time.