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. 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-compatible.