Vintage Late 70's MXR dyna comp
 
Model MX-102 Block Logo
 
The compression effect helps to smooth out differences in volume between notes, such as in quick runs played by chicken picking. It also fattens up the sound, giving more body and sustain to lead guitar lines, and can be used as a booster. Its "thickening" effect is especially appreciated in Nashville, where many country music guitar players who use Fender Telecasters use it to thicken the otherwise thin sound of the single-coil guitar; besides, the compressor's lessening of the initial attack "gives the effect of a pedal steel player working his volume pedal". 
 
The output knob controls the volume
 
The sensitivity knob controls the ratio (highest sensitivity is maximum compression)
 
Notable users include David Gilmour, Adrian Belew, John Frusciante, & more!