Double Data Rate SDRAM was an evolutionary improvement over SDRAM. It allowed data to be transferred twice during every cycle. This effectively doubled the frequency of the memory; DDR 266 memory actually works at 133 MHz.

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It is important to note that the term is "DDR 266" and not "DDR 266 MHz." Another mode of notation is by referring to the peak data transfer speeds. So a DDR 266 module is also referred to as PC 2100. since it can transfer 2100 Megabytes per second. DDR modules have 184 contacts. and are not backward-eompatible.