Open G is tuned D-G-D-G-B-D; open A raises each of those notes a whole-step (2 frets) to E-A-E-A-C#-E. During the 1920s and 1930s, much of the sheet music written for lap steel utilized open A tuning as the de facto standard tuning for the instrument.

Open D is tuned D-A-D-F#-A-D, and open E is a whole-step higher: E-B-E-G#-B-E. Joe Perry of Aerosmith uses Open E on his electric lap steel. David Lindley is another player who uses enharmonic variations of these tunings.

Bluegrass and Country Dobro players using a square-neck instrument tend to favor an altered G tuning, often called "High-G", where the 6th string is tuned up to "G" instead of down to "D", and the 5th string is also tuned up, to B: G-B-D-G-B-D. They also sometimes raise it up to "High-A": A-C#-E-A-C#-E. These are examples of tunings possible on a lap steel that would cause serious damage if attempted on a round-neck resonator or standard guitar.

Dobro players also generally use a set of strings with different gauges than those used on standard electric or acoustic guitars to help them to project more sound and to achieve their higher tunings.

Many Western Swing steel players, and some Old-Time Country steel players, use a C6 tuning. There is no "standard" C6 tuning; one popular one is C-E-G-A-C-E (C E G is a C major chord and A C E is an A minor chord). This tuning is a good one for copping Don Helms' steel licks off old Hank Williams records, although Helms used a steel with legs (a "console steel"), with two necks having 8 strings each; Helms actually used an E13 tuning, which adds the 7th (D) and the 13th (C#) to the E tuning, making it B-D-E-G#-B-C#-E-G#, low to high. An extended C6/FMaj7 is used by Western Swing pedal steel guitarists on their 10-string pedal steels. This tuning, "C-F-A-C-E-G-A-C-E-G", is difficult to achieve on the 6-string steel but a subset thereof is achieved as previously mentioned. A6 is a commonly-used alternate for C6.

Recommended string gauges: .36 .30 .26 .24 .18(p) .16(p)

The E7 tuning is used by many players, especially those who starting learning with the Mel Bay Steel Guitar Method instructional books. The E7 tuning in those books is spelled either B-D-E-G#-B-E or with the 6th string lowered to the tonic E: E-D-E-G#-B-E. Note the similarity of this second tuning to the open E tuning above: the only difference is the 5th string, which is lowered from the tonic E to the 7th note in the key of E, which is D.

There are many other tunings used by players. Pedal Steel guitarists switching over to lap steel often bring over a modified version of the 10-string E9 tuning that is the standard for Country pedal steel; pedal steels, and a few non-pedal "console steels" actually have multiple necks, each in a different tuning, and very often on a pedal steel the 2 main necks will be in E9 and C6 tunings. As noted under the C6 tuning, an A6 tuning is also used.

( from WidipediA: Lap Steel )