The seventh and final mode of the major scale is the locrian mode. In the key of C, a locrian scale is built on B, and consists of "B, C, D, E, F, G, A". The seventh chord built on this scale ("B D F A") is a half diminished seventh chord, Bm7b5. This symbol comes from the fact that this chord is similar to a Bm7, except that the fifth is lowered by a half step. The classical symbol for this chord is a circle with a "/" through it. The locrian scale can be used over a half diminished (also called a minor seven flat five) chord, but the second step is somewhat dissonant and is sometimes considered an avoid note.