Breath controller (a.k.a controller number 2) is a MIDI continuous controller command set aside for parameters lending themselves to breath control. To fully understand why something seemingly this obscure has a designated controller number, one has to go back to the early days of MIDI, when the Yamaha DX-7 came out. The DX-7 utilized a breath control device to add realism to certain types of sounds such as brass and woodwind instruments. The breath controller itself was a small device that connected to a proprietary port on the back of the DX-7. A musician could insert it into the mouth (like a whistle) and blow through it. The air velocity was measured and turned into control data inside the DX. The control data could then be used to open a filter or some other assigned parameter to manipulate the sound by the player without having to do anything special with his/her hands or feet. MIDI was in its infancy at the time, and the DX-7 was an extremely popular and groundbreaking instrument in a number of respects. As such it seemed likely that breath control would become a common way of manipulating synth parameters in real time, and so it made sense for a controller to have this function. In reality there is nothing unique about controller #2 compared to most of the other controllers. It can be used for any common continuous controller command so long as you set up the transmitting and receiving devices accordingly. You will simply see it referred to as breath controller pretty frequently in documentation. While breath controllers aren’t as popular today as we once thought they would be, there are quite a few players who use them.