A MIDI keyboard is an electronic piano keyboard that can be connected to any electronic musical instrument that has a MIDI IN jack. By stringing one electronic instrument to another in a chain (connecting cables from the MIDI OUT jack of one instrument to the MIDI IN jack of another instrument), you can play up to 16 musical instruments at once with independent control over the various parameters of each one of them.

The musician in this way directly controls the keyboard, and through the keyboard the other instruments play each other, giving the musician indirect control over all of them. A MIDI keyboard is also often referred to as a MIDI controller keyboard, because the keyboard is the master controller of a string of electronic instruments. Ironically, in many cases the keyboard itself can produce no sound – it can only play the sounds of other instruments (and the instruments themselves cannot be directly played by a human, only by each other through the use of MIDI signals).

If you are a beginner considering buying your first keyboard (and if you want something cheap but usable) make sure that it is fully “touch sensitive”:

(1) Your keyboard should support “MIDI Channel Pressure”: If your press a key and hold it down, varying the pressure of your finger on the key should create MIDI channel pressure messages that will be sent to your MIDI external hardware. Depending on the type of hardware you're using, this channel pressure can be sued to manipulate parameters such as the tremulo and the vibrato. If you don't know what these are, just try pressing down on the keyboard (BEFORE you buy it!), hold it there for a minute, vary the pressure that your finger is applying, and listen for the difference in the sound of the note.

Your keyboard should also produce MIDI Note Velocity messages – the “note velocity” should increase the harder you press down on the key. Depending on the system you're using, this will increase either the note's volume or its brightness. Most MIDI keyboards support this feature, but do confirm this with your dealer.

Ask your dealer about both of the above functions – if you simply ask him whether or not the keyboard is “touch sensitive”, of course he'll say “yes” – after all, when you touch the keyboard it makes a sound. But this is not enough for effective MIDI composing.