A DIMM is a Dual Inline Memory Module.

A DIMM is a small circuit board designed to hold a set of RAM chips.

A DIMM has a 64-bit data bus, which allows DIMM's to be connected one at a time to a CPU with a 64-bit data bus.
DIMM Sizes

DIMM's are available in several different physical sizes.

Contact Count Usage
72 SODIMM
144 SODIMM
168 SDRAM
184 DDR SDRAM
240 DDR2 SDRAM

DIMM contacts are single sided, which means that a DIMM with 72 contacts will have 36 contacts on each side.
DIMM Types

DIMM modules are sold according to clock speed (MHz), bus speed (mega-transfers per second), and transfer rate (megabytes per second.
Standard SDR SDRAM DIMM's
DIMM Module Chip Type Clock Speed Bus Speed Transfer Rate
PC66 10ns 66 66 533
PC100 8ns 100 100 800
PC133 7.5/7ns 133 133 1,066
Standard DDR SDRAM DIMM's
DIMM Module Chip Type Clock Speed Bus Speed Transfer Rate
PC1600 DDR200 100 200 1,600
PC2100 DDR266 133 266 2,133
PC2400 DDR300 150 300 2,400
PC2700 DDR333 166 333 2,667
PC3000 DDR366 183 366 2,933
PC3200 DDR400 200 400 3,200
PC3500 DDR433 216 433 3,466
PC3700 DDR466 233 466 3,733
PC4000 DDR500 250 500 4,000
PC4300 DDR533 266 533 4,266
Standard DDR2 SDRAM DIMM's
DIMM Module Chip Type Clock Speed Bus Speed Transfer Rate
PC2-3200 DDR2-400 200 400 3,200
PC2-4300 DDR2-533 266 533 4,266
PC2-5400 DDR2-667 333 667 5,333
PC2-6400 DDR2-800 400 800 6,400
168 Contact DIMM's

The earliest DIMM's were of the 168 contact variety. These DIMM's have come in a variety of configurations. It is possible to determine the configuration of the DIMM by examining the placement of notches on the bottom of the DIMM.
DIMM Notch Key One

Position

Meaning

1

Reserved

2

Buffered

3

Unbuffered
DIMM Notch Key Two

Position

Meaning

1

5.0V

2

3.3V

3

Reserved

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