E9 tuning is a common tuning for steel guitar

Steel guitar

Steel guitar is a type of guitar or the method of playing the instrument. Developed in Hawaii in the late 19th and early 20th centuries, a steel guitar is usually positioned horizontally; strings are plucked with one hand, while the other hand changes the pitch of one or more strings with the use...

 necks of more than six strings. In particular, it is the most common tuning for the far neck on a two-neck table steel guitar

Table steel guitar

The console steel guitar is a type of electric steel guitar intermediate between the lap steel guitar and the pedal steel guitar. Console steel guitars typically have multiple necks and/or more than six strings per neck, and are too large to be easily played in lap steel fashion.They are...

 or pedal steel guitar

Pedal steel guitar

The pedal steel guitar is a type of electric guitar that uses a metal bar to "fret" or shorten the length of the strings, rather than fingers on strings as with a conventional guitar. Unlike other types of steel guitar, it also uses pedals and knee levers to affect the pitch, hence the name "pedal"...

, most often combined with C6 tuning

C6 tuning

C6 tuning is the most common tuning for electric steel guitar, both on single and multiple neck instruments. On a twin-neck, the most common tuning is C6 tuning on the near neck and E9 tuning on the far neck....

 for the near neck, and also a popular tuning for single neck instruments of eight or more strings.

The E9 tuning has evolved to support optimal chord and scale patterns across a single fret on the 10-string pedal steel guitar.

Corresponding tunings for a six string lap steel guitar

Lap steel guitar

The lap steel guitar is a type of steel guitar, an instrument derived from and similar to the guitar. The player changes pitch by pressing a metal or glass bar against the strings instead of by pressing strings against the fingerboard....

 are the E6 tuning E-G-B-C-E-G, or E7 tuning B-D-E-G-B-E.

A popular E9 tuning for eight string table steel guitar

Table steel guitar

The console steel guitar is a type of electric steel guitar intermediate between the lap steel guitar and the pedal steel guitar. Console steel guitars typically have multiple necks and/or more than six strings per neck, and are too large to be easily played in lap steel fashion.They are...

 is the western swing

Western swing

Western swing music is a subgenre of American country music that originated in the late 1920s in the West and South among the region's Western string bands...

tuning E-G-B-D-F-G-B-E, low to high and near to far.

The standard Nashville E9 tuning for ten string pedal steel guitar

Pedal steel guitar

The pedal steel guitar is a type of electric guitar that uses a metal bar to "fret" or shorten the length of the strings, rather than fingers on strings as with a conventional guitar. Unlike other types of steel guitar, it also uses pedals and knee levers to affect the pitch, hence the name "pedal"...

is B-D-E-F-G-B-E-G-D-F.