What is PCIe? Understanding PCIe Slots, Cards and Lanes

Just by looking at the picture of the board on the linked website, the second slot says PCI 16_2, which would say to me that it's a PCIe 3 x16

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what is a mail slot NVIDIA's GeForce RTX 3090 Graphics Card Is An Absolute Unit – Triple-Slot Monster Pictured Inside Standard ATX Case The first picture of

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