1996: Double Data Rate (DDR) SDRAM
As a successor to SDRAM, the DDR SDRAM standard was developed at JEDEC between 1996 and 2000. DDR (1) debuted at bus clocks and internal speeds of 133, 166 and 200 MHz and version numbers DDR-266, DDR-333 and DDR-400. The first DDR SDRAM specification was released in June 2000. DDR memory devices are not backwards compatible with SDRAM.
The second generation, DDR2, followed in 2003 and was available with clock rates of 200, 266, 333, 400 and 533 MHz (DDR2-400, DDR2-533, DDR2-667, DDR2-800, DDR2-1066). The current generation DDR3 was generally released in 2006 and is available in400, 533, 667 and 800 MHz versions (DDR3-800, DDR3-1066, DDR3-1333 and DDR3-1600). DDR4 devices are currently expected to be released in 2014.