AMD Socket 939
When AMD introduced its Athlon 64 processor, it divided the platform using two different sockets: Socket 754 supported single-channel DDR400 memory and low-cost to mainstream processors. Socket 940 was used for the upper mainstream and the high-end, supporting dual channel DDR400 memory and faster processors.
However, AMD quickly decided to drop socket 940 (which is the same socket the professional Opteron processors used at that time) in favor of socket 939. The latter didn't require registered DDR memory and thus allowed for cheaper high-end systems.
First socket 939 systems used AMD, SiS, VIA and Nvidia chipsets with AGP graphics, but ATI, Nvidia, SiS and VIA quickly released PCI Express chipsets as the new multi-purpose interface became important for graphics.
Socket 939 is outdated and was replaced by the 940 pin socket AM2, which supports DDR2 memory instead of DDR400.