While the term Locrian mode has been used by several early music theorists, it was first applied to chant in the 18th century. In modern terms, it is most easily described as the scale from B to B using only the white notes of the piano. In order to construct the scale starting on other notes, follow the pattern ascending from tonic: half step – whole step – whole step – half step – whole step – whole step – whole step.
A handout showing the mode starting from all twelve notes is available here.
The Locrian mode is considered a minor mode because of the minor third above the tonic. It differs from the natural minor scale by having lowered second and fifth degrees. The ancient Greek Locrian mode in the diatonic genus resembles the Mixolydian mode rather than the modern Locrian mode.
The presence of a diminished fifth in the tonic triad (B-F) has resulted in few composers actually using the mode for more than brief passages. Claude Debussy’s includes three extended passages in the Locrian mode in his composition Jeux.