If you have ever plugged in a graphics card, an NVMe SSD, or a high-speed network adapter, you have used PCI Express (PCIe). But beneath the familiar slot shapes lies a complex, evolving standard specification that dictates the speed, power, and future of nearly every high-performance computer.

Let’s break down what the PCIe specification actually is, how versioning works, and why it matters for system architects and hardware engineers. The PCIe specification is a formal technical document maintained by the PCI-SIG (Peripheral Component Interconnect Special Interest Group). This group—comprising companies like Intel, AMD, NVIDIA, and Arm—defines everything from the physical size of the connector to the low-level packet protocols.