In the words of Boss:

First released in 1991 and in continuous production ever since, the MT-2 Metal Zone is loved by guitarists everywhere for its thick, rich distortion and distinctive mid-focused character. Capable of producing a versatile range of heavy tones, the MT-2s innovative dual-stage gain circuit has influenced legions of pedal designers through the years. Another standout feature is its groundbreaking three-band EQ with parametric midrange, which provides deep sound-shaping ability for covering numerous high-gain styles. The pedal was also one of the first BOSS compacts with dual-concentric knobs, offering expanded control without increasing size.