What's the difference between single-DIN and double-DIN head units?

Single-DIN units occupy a standard 2-inch-tall dash slot and are often 'mechless' Bluetooth receivers with no screen (or a small floating display), while double-DIN units are twice as tall and typically include a 6.8-to-10-inch touchscreen. Check your vehicle's factory opening before buying — some cars only accept one size without an adapter kit.