In this guide
USB-A is a flat, rectangular connector used on host devices like computers and chargers, while USB-B is a squarer connector with beveled corners found on peripherals like printers and scanners — the two shapes are not interchangeable.
- You connect USB-A (host) to USB-B (peripheral) using a standard A-to-B cable
- Both connectors match the same USB version (e.g., both USB 3.0)
- Cable is inserted in the correct orientation — neither connector is reversible
- You try to plug a USB-A connector into a USB-B port or vice versa
- You mix USB versions without understanding the speed downgrade
- The connector is forced in the wrong orientation
Physical Shape and Where Each Connector Appears
USB-A has a thin, flat rectangular shape — you see it on laptop ports, wall chargers, and USB hubs. USB-B has a roughly square body with two beveled top corners, giving it a distinctive chunky look. It appears on the device end of printer cables, external audio interfaces, and older external hard drives.
Because the shapes are completely different, you cannot accidentally plug one into the other. A USB-A plug physically will not seat in a USB-B socket, and the reverse is equally true.
Host vs. Device Convention
USB was designed around a host-to-device model. The host (your PC, laptop, or charger) always carries the USB-A port. The peripheral (printer, scanner, DAC) always carries the USB-B port. A standard USB-A to USB-B cable follows this rule from end to end, which is why the connector shapes are intentionally different — it prevents you from wiring two hosts together.
USB Version and Speed Compatibility
| Setup | Works? | Need | Note |
|---|---|---|---|
| USB-A 2.0 to USB-B 2.0 | Yes | Standard A-to-B cable | Up to 480 Mbps |
| USB-A 3.0 to USB-B 3.0 | Yes | Blue-keyed A-to-B 3.0 cable | Up to 5–10 Gbps |
| USB-A 3.0 to USB-B 2.0 | Yes | Compatible cable | Speed drops to 480 Mbps |
| USB-A to USB-B (wrong orientation) | No | Flip connector | Neither end is reversible |
USB 3.0 ports and cables are typically identified by a blue plastic insert inside the connector. Always check the USB version on both your host port and your peripheral to know what speed to expect. The USB-IF official connector specifications detail exactly how each version differs physically and electrically.
How USB-A and USB-B Compare to Newer Connectors
Both USB-A and USB-B have largely been succeeded by USB-C on modern devices. USB-C is oval, reversible, and capable of carrying video, data, and power over a single cable. If you are connecting a newer peripheral that has a USB-C port, the older USB-A and USB-B connectors are not involved. You can learn more about what USB-C can carry in our guide on whether USB-C supports video, or check our article on connecting USB-C to HDMI if you need display output.
Quick Identification Tips
- USB-A: flat rectangle, wide end goes into the host
- USB-B: square with beveled top corners, goes into the peripheral
- Blue plastic inside = USB 3.0 or higher
- Neither connector is reversible — check orientation before inserting
Bottom line: USB-A and USB-B serve opposite ends of the same cable — A for the host, B for the peripheral. Their shapes make it impossible to connect them incorrectly, as long as you match the right cable to the right ports and verify USB version if speed matters.