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[Title] Notch [Description] 
Notches are cut outs that are located on the bottom of a memory module. Each memory type (SDRAM, DDR, DDR2, and DDR3) has a unique notch location for DIMMs and SODIMMs in order to prevent the wrong type of memory from being installed in the system. Notches also help ensure that the memory module is installed correctly.
Crucial.com - Memory terms glossary
Description