DIMM is an abbreviation for Dual In-line Memory Module, and is a form of RAM for usage in personal computers. DIMMs began to replace SIMMs as the predominant type of memory module when Intel's Pentium processors were in the mainstream market.

The most common types of DIMMs are:

• 72-pin-DIMMs, used for SO DIMM
• 144-pin-DIMMs, used for SO DIMM
• 168-pin-DIMMs, used for SDRAM
• 184-pin-DIMMs, used for DDR SDRAM
• 240-pin-DIMMs, to be used for DDR-II SDRAM

There are 2 notch keys in 168-pin-DIMMs, each has three different notch positions which represents three different types:

The first notch is DRAM key position. It represents RFU, buffered and unbuffered.

The second notch is voltage key position. It represents 5.0, 3.3 and Reserved.

The upper one in the photo is an unbuffered 3.3V 168-pin-DIMM.