Memory is available in various physical packaging. Roughly in order of their appearance, the major types of DRAM packaging include:

DIP (Dual Inline Pin Package)

This package comprises a rectangular chip with a row of pins down each long side, making it resemble an insect. DIP was the most common DRAM package used in PCs through early 386 models. DIP chips were produced in Page Mode and Fast Page Mode, and are long obsolete. DIP packaging was also used for L2 cache memory on most 486 and some Pentium-class motherboards. DIP DRAM is useless nowadays.

SIPP (Single Inline Pin Package)

This package turns a DIP chip on its side and extends all leads straight out one side, parallel to the plane of the chip. SIPPs were intended to allow memory to be installed more densely, and were used in a few 386SX systems, but they never caught on. SIPPs were produced in Page Mode and Fast Page Mode form, and are long obsolete.

SIMM (Single Inline Memory Module)

This package mounts multiple individual DRAM DIP chips on a small circuit board with a card-edge connector designed to fit a socket on the motherboard. Mainstream SIMMs have been manufactured in two form factors:

30-pin

These SIMMs were used in a few 286 systems, most 386 systems, and some 486 systems, and were produced in Page Mode and Fast Page Mode form. Although they are still available, 30-pin SIMMs are obsolete. If you tear down an old system, any 30-pin SIMMs you salvage are too small and too slow to be useful. However, some laser printers do use them.

72-pin

These SIMMs were used in some 386 systems, most 486 systems, and nearly all Pentium-class systems built before the advent of DIMMs. 72-pin SIMMs were produced in Fast Page Mode, EDO form, and BEDO form. When tearing down old systems, 72-pin SIMMs may be worth salvaging, as they can be used to expand the memory on a late-model Pentium or Pentium Pro system or to expand the memory in some laser printers.

DIMM (Dual Inline Memory Module)

DIMMs are dual-side modules that use connectors on both sides of the circuit board. SDR-SDRAM DIMMs have 168 pins, but SDR-SDRAM is also available in 100- and 144-pin DIMMs. DDR-SDRAM is packaged in 184-pin DIMMs, which are physically similar to standard 168-pin SDR-SDRAM DIMMs, but have additional pins and different keying notch positions to prevent them from being interchanged. DDR-II DIMMs are similar to DDR DIMMs, but use a 232-pin connector. Only SDR-SDRAM, DDR-SDRAM, and EDO are commonly packaged as DIMMs.

SODIMM (Small Outline DIMM)

A special package used in notebook computers and on some video adapters.

RIMM

A Rambus RDRAM module. RIMM is a trade name rather than an acronym. RIMMs are physically similar to standard SDRAM DIMMs, except that the keying notches are in different locations. RDRAM is available in 168-pin and 184-pin modules. Early RDRAM motherboards used 168-pin RIMMs. Most current RDRAM motherboards use 184-pin RIMMs.

The major difference between these DIMMs, other than the number of pins, is the location of the keying notches. SDRAM DIMMs use two notches, one centered and one offset. DDR-SDRAM DIMMs use only one offset keying notch. The number and position of these keying notches ensure that only the proper memory type can be installed in a slot and that the module is oriented correctly. Rambus RIMMs use similar physical packaging, but with the keying notches in different locations. Rambus RIMMs also cover the individual chips with a metal shroud designed to dissipate heat.