S1A: PCIPCIe Gen 2, 2 PCI and 2 PCIe slots

The IO to PCIE spacing for slot '1' is standard between motherboard sizes as I understand it The rest then follow 'down' the board

A PCIe x16 slot has a physical length of approximately 89mm and has a single notch on the connector PCIe x16 devices are not compatible with smaller PCIe slots

pcie slot pinout Expansion card riser specifications ; Slot 1, Riser 1, Low Profile, Half length, x16 ; Slot 3, Riser 2c, Low Profile, Half length, x16

pcie gen 4 in gen 3 slot slot can actually share the same size an x16 slot If you are a DIY PC builder, It is