What is the IAC Driver on a Mac?

The IAC Driver is a virtual MIDI cable built into macOS. It lets one app send MIDI to another on the same Mac, with no hardware between them: one app sends to an IAC bus, and the other listens to it. It does nothing until it is online. Open Audio MIDI Setup, choose Window, Show MIDI Studio, double-click IAC Driver, and select Device is online.

The steps are Apple's, from its Audio MIDI Setup guide. IAC stands for Inter-Application Communication.

On this page: 8 sections

Steps
Turn the IAC Driver on
Know what a bus is
Use it for the jobs it is good at
Avoid a MIDI loop
Use it with InputConfig
The short version
Questions

Steps

Open Audio MIDI Setup and choose Window, Show MIDI Studio.
Double-click the IAC Driver icon.
Select Device is online.
Add a bus with the Add button under the list of ports, and name it if you like.
Click Apply, then choose that bus as the output in one app and the input in the other.

Turn the IAC Driver on

You: In MIDI Studio, double-click IAC Driver. In its Properties window, select Device is online. Add a bus with the Add button below the list of ports, then click Apply.
macOS: Creates a MIDI port that every app on the Mac can send to and listen to.
Check: The bus shows up in your music apps as both a MIDI output and a MIDI input.

One bus is enough to start: one app uses it as a destination and another uses it as a source. Apple's page is Transfer MIDI information between apps.

InputConfig is free on the Mac App Store. macOS 14 or later, no account, no tracking, open source.

Know what a bus is

Apple's guide calls each connection a bus, and notes that it is sometimes called a port. You can add more than one, rename one by double-clicking it, and remove one you no longer use. A second bus keeps two jobs apart, such as notes on one and sync on the other.

Name each bus after its job, and the right one is easy to pick in every app.

Use it for the jobs it is good at

One music app playing another. A keyboard app sends its notes to a synthesizer app, which is Apple's own example.
Sync between apps. A DAW sends its MIDI clock or transport to a second app.
A DAW driving shortcuts. A DAW sends Start and Stop to a bus, and a mapper turns them into actions in another app.

If two apps on one Mac need to pass MIDI, an IAC bus is the cable between them.

Avoid a MIDI loop

An app that listens to a bus and also sends to it can hear its own output and send it round again. Give each app one direction on a bus: one sends, the other receives. If notes repeat or stick the moment you turn the driver on, an app is listening to the bus it sends to.

Use it with InputConfig

InputConfig, the free Mac app I make, uses MIDI in both directions, and the IAC Driver is needed for only one of them.

Into InputConfig from a DAW. Send the DAW's MIDI sync to an IAC bus, and InputConfig hears Start, Continue and Stop, so pressing play in the DAW can start a recording in another app.
Out of InputConfig. No IAC Driver needed. The app has its own virtual port, named InputConfig, that music apps see whenever it runs.

InputConfig sends through its own port and listens through an IAC bus. MIDI Start and Stop as Mac shortcuts and MIDI output have the steps.

The short version

What it is. A virtual MIDI cable between apps on one Mac.
Where it is. Audio MIDI Setup, Window, Show MIDI Studio, IAC Driver.
How to turn it on. Device is online, add a bus, Apply.