EMZETT.USB-A
In short: The classic, rectangular USB connector type that only fits into the jack in one orientation — the standard connector for USB peripherals for decades.
In more detail: USB-A jacks are found on practically every PC and many peripherals. Unlike USB-C, USB-A isn’t “flip-proof” (reversible) — a well-known everyday annoyance when plugging it in.
In Depth
Origin and adoption
USB-A is the original connector shape, introduced in 1996 with the first USB standard, and remains one of the most widely used USB connectors to this day — practically every desktop PC, many laptops, chargers, car dashboards and countless peripherals have at least one USB-A jack. Over the decades, the connector shape itself has barely changed, enabling remarkable backward compatibility: a USB-A plug from 1998 fits mechanically without trouble into a current USB-A jack, even though the actually achievable speed is limited by the older of the two devices involved.
The characteristic connector shape
The plug is deliberately shaped asymmetrically: it only fits into the jack in one orientation, which on the one hand prevents it from being plugged in the wrong way round or with swapped contacts, but on the other hand causes the well-known “three-try problem” — the correct orientation is often not immediately obvious from outside, which is why users reflexively flip the plug, flip it again, and only get the originally correct orientation on the third try. USB-C solves this problem with a fully symmetrical, reversible connector shape.
Speed generations
Electrically, USB-A supports all common USB speed generations up to USB 3.2 Gen 2 (up to 10 Gbit/s). To distinguish them, the inside of faster USB-A jacks is often coloured blue (USB 3.x) or turquoise (USB 3.2 Gen 2) instead of black (USB 2.0) — a purely visual cue with no technical meaning, but one that helps quickly identify the right jack. The pin assignment was extended with USB 3.0 (additional contacts for the higher speed), so a USB 3.0 plug still fits into an old USB 2.0 jack, but then only runs at USB 2.0 speed.
Limits compared to USB-C
Because of its connector shape, USB-A is limited compared to USB-C in power delivery and maximum data rate. The newest, very fast USB4 standards (up to 40 Gbit/s) as well as USB Power Delivery at high wattage (up to 240 watts, enough for many laptop power adapters) are specified exclusively over USB-C — USB-A simply can’t physically provide enough contact area for the additional signalling and power capacity needed for that. For this reason, more and more modern laptops (MacBooks since 2016 leading the way) forgo USB-A entirely and only include USB-C ports, which requires an adapter for older peripherals.