To further clarify Mike's excellent response.

CC 7 (track volume) is akin to the volume knob on a guitar amp or an electronic keyboard amp. Set and forget.

CC 11 (expression) is momentary volume changes, like a volume pedal on an organ, guitar, or electronic keyboard between the instrument and the amp

The third volume control is velocity, which is called velocity because early synths used piano style keyboards. The less time difference between when the key was struck and when it hit the bottom was the speed (velocity) represents how hard the key was hit. The harder you hit the key, the louder the note on a piano and thus velocity is a third volume control which mimics how hard you hit the keyboard or how hard you pick the string and is used to determine individual note volumes.

I have a complete list of standard MIDI continuous controllers here: http://www.nortonmusic.com/midi_cc.html

All these controllers help get life-like expression out of MIDI sequences, thus turn 'empty notes' into music.

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Bob "Notes" Norton smile Norton Music
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