B♭ tuning

B♭ tuning, or A♯ tuning, is a method of guitar tuning (and stringed instruments per se) in which all strings on a six-stringed instrument, most often guitar, are tuned down by 3 steps. For example, standard guitar tuning is E A D G B E. B♭ tuning starts by tuning the lowest string on a guitar E, to B♭ and then tuning all strings down in the same interval of 3 steps down.

Strings on a guitar tuned to B♭ are B♭ E♭ A♭ D♭ F B♭

Seven-string guitars achieve B and B♭ tuning because they have a lower B string below the E string, which is the lowest string on a conventional guitar.