The first step into DDR (double data rate) SODIMMs came with PC1600, also known as DDR-200. The internal clock rate of 100 MHz yielded an actual data rate of 200 MT/s (200 million transfers per second). DDR SODIMMs scaled up to include DDR-266, -333, and -400 specifications, with DDR-400 (PC3200) sporting a peak transfer rate of 3200 MB/s. The module shown here is a 64 MB PC2100 unit. Note the two edge connector keys.