Each note on a MIDI controller keyboard has a corresponding MIDI note number. These numbers are standardized across keyboards and other MIDI devices. This standardization ensures that all notes will have the same number throughout different controllers and programs. There are 128 possible MIDI note numbers, beginning with 0 and ending with 127. Middle C is designated with number 60. The C five octaves below middle C is number 0, and the G five octaves above is number 127.
In musical terms, transposing refers to changing the key of a piece of music. A Western musical octave is divided into 12 equally spaced intervals, called semitones. Increasing a pitch by 12 semitones arrives at the same note, an octave higher. Likewise, decreasing 12 semitones transposes an octave lower. You can transpose to any key, from any other key simply by adding or subtracting a fixed number of semitones from all notes involved.
In the world of MIDI, transposing involves adding or subtracting a constant number from all MIDI notes on the keyboard. Each consecutive MIDI note number corresponds to a semitone change in pitch. Most MIDI keyboards have an octave "Transpose" button, for shifting the range of the keyboard by adding or subtracting 12 from each key's corresponding MIDI note number. A more versatile "Transpose" button allows you to transpose in increments other than 12. For example, if you transpose from C to A, you subtract 3 from all MIDI note values. Middle C moves from 60 to 57, E-flat becomes the new middle C (from 63 to 60), and all other notes shift as well.
The act of transposition happens on the keyboard itself. Therefore, if you are using your MIDI keyboard to connect to other devices or programs, they will receive the transposed MIDI note numbers instead of the keyboard's original settings. If you want, most digital audio workstation (DAW) programs allow you to manipulate MIDI note data after it has been recorded. In this way, you can transpose after playing on the keyboard. This can be helpful if you are familiar with the sound of an actual piano and the keyboard-based transposition becomes a distraction.