DIMMs have 168 pins in two (or dual) rows of contacts; one on each side of the card. With the additional pins a computer can retrieve information from DIMMs, 64 bits at a time instead of the 32- or 16-bit transfers that are usual with SIMMs.
Some of the physical differences between 168-pin DIMMs and 72-pin SIMMs include: the length of module, the number of notches on the module, and the way the module installs in the socket. Another difference is that many 72-pin SIMMs install at a slight angle, whereas 168-pin DIMMs install straight into the memory socket and remain completely vertical in relation to the system motherboard. Importantly, and unlike SIMMs, DIMMs can be used singly and it is typical for a modern PC to provide just one or two DIMM slots.
The 3.3 volt unbuffered DIMM emerged as the favoured standard. This allows for SDRAM, BEDO, EDO and FPM DRAM compatibility as well as x64 and x72 modules with parity and x72 and x80 modules with ECC and takes advantage of the left key position to establish a positive interlock so that the correct DIMMs are inserted in the correct position.