Alternate Mode, often written Alt Mode, is a capability of the USB-C connector that allows some of its high speed pins to be reassigned to carry a different protocol instead of standard USB data. This is negotiated automatically when a compatible cable and device are connected.
The best known use is video output, through DisplayPort Alt Mode or the less common HDMI Alt Mode, which is how many laptops and phones can drive an external monitor from a single USB-C port without a separate video connector. Thunderbolt also relies on a form of Alternate Mode to share the same physical port as USB.
Support for Alternate Mode is not guaranteed on every USB-C port, since it depends on the internal wiring of the device, so a port that charges and transfers data perfectly well may still not output video. Checking a device's specifications for explicit DisplayPort or HDMI Alt Mode support is the way to be sure. See our guide to USB video output for more.
