OS X has a neat built-in feature that lets two MIDI programs on the same machine talk to each other, making all kinds of interesting projects possible. It’s called the IAC Driver.

Note: For Windows XP and newer (Windows 10, etc.): LoopBe1 is a free third-party driver that does the same thing. Or, for Windows XP and earlier (Windows 2000, etc.): use MIDI Yoke.

Back to OS X—

Step 1. Find and launch the Audio MIDI Setup utility:

Step 2. Go to MIDI Devices and find the IAC Driver’s icon. If you’ve never used it before, it’ll be grayed out:

Step 3. Turn it on by checking the Device is online box:

Step 4. Now you can go to the MIDI preferences of any program you like and treat the IAC Driver as a MIDI input or output: