Slot 1 refers to the physical and electrical specification for the connector used by some of Intel's microprocessors, including the Celeron, Pentium II and the Pentium III. Both single and dual processor configurations were implemented.
Slot 1 was a departure from the square Zero Insertion Force PGA/SPGA sockets used for the Pentium and earlier processors. Instead, the processor is mounted on a Single Edge Contact Cartridge (SECC), much like a PCI slot, but with a 242-lead edge-connector.
The Slot 1 specification allows for higher bus rates than Socket 7. Slot 1 motherboards use the GTL+ bus protocol.
Some Pentium-II 350MHz and 450MHz processors, and all slot-1 Pentium-III's, came in the improved SECC2 variety. The SECC2 package varied from the original SECC package by removal of the heat-plate connected to the CPU via a thermal interface, used less raw material, was lighter, and could be produced at lower cost.
It should be noted that while SECC2 retention clips will hold a SECC package, SECC clips will not hold an SECC2 package. The Slot A standard used by AMD was mechanically identical but electrically incompatible.