The MXL DX-2 is the first variable dynamic dual capsule microphone in the world that can now fully customize the sound by mixing one capsule with the other. The design of the super super cardioid capsule (Capsule One), is tuned to capture the warmth and fullness of a musical instrument, while the smaller cardioid capsule (Capsule 2) offers an excellent medium level and high-end details.
The front of the microphone is a design that allows the capsules to be close to the source for optimal sound capture, while the back has the button that allows interaction between the 2 capsules, which facilitates adjustments. 
For more information: http://www.mxlmics.com/microphones/studio/DX-2/