SIMM memory modules look like this:
And they are placed in a "SIMM socket" on the motherboard of the computer. The following picture shows a SIMM slot:
SIMM's, come in 30 and 72 pin sizes the latter now becoming the standard. Older boards used by Intel 386, and some 486 machines have 30 pin sockets. The 30 pin SIMM module was designed for 8-bit data flow, and are generally placed in groups of four to give a 32-bit data path. (4x8 = 32). This would mean that with 32 bit operation would be 4, 30 pin SIMM's at a time. Thus the 72 pin SIMM was designed for 32-pin operation, reducing the number of sockets required although this had to be off-set with a 50% increase in physical length. To handle 64-bit operation the 72 pin SIMM would need to 'pair-up', once again, causing a space problem once more on the motherboard. One answer was the DIMM(dual in-line memory module).
Most Pentium motherboard now incorporate two, or more slots, for a 168 pinn DIMM module. DIMMs look similar to the older SIMM but differ in a key area. The DIMM module uses both sides of the chip with 168 pin version having 84 pins on each side. Each pin is completely independent of the others, and makes a separate electrical contact. The density of the DIMM memory module is increased with only the slightest increase in the module's physical size.
DIMM's also fit differently into the slot. The DIMM module is slid into 'channels' on the two edges of the socket. They are then pressed into the socket until they 'click' home.