In the beginning of computer history, there was no difference between the internal and external clock speed. A 386 DX 25 worked at 25 MHz processor clock and was able to communicate at the same 25 MHz with its page mode DRAM. In the beginning of the 90s it became obvious that processor speeds would rise faster than clock speeds of the main memory (best example was the 486 DX 50) and upcoming synchronous bus systems. Since the performance losses at lower clocked memory were tolerable, Intel introduced a new series of processors: The 486 DX2. The "2" means that the processor is clocked twice as high as the memory/bus. A 486 DX2 - 66 worked at 33 MHz memory/bus and 66 MHz processor clock.