A C-RIMM (or Continuity-RIMM) is a pass-through device used on motherboards with Rambus DRAM modules (RDRAM). Rambus technology uses very fast clock signals to pass data between the memory and processor, so the electrical signals demand that every Rambus slot be filled. If you don't buy enough RDRAM to fill every RIMM slot, you'd use C-RIMMs in slots that do not have Rambus modules installed. C-RIMMs do not have any memory devices; they serve to maintain the continuity of Rambus channel signals. When you upgrade the motherboard with additional Rambus modules, the C-RIMMs in those locations can be removed and replaced with real RIMMs.