The precise reason for this is straightforward: 486 processors had a 32-bit data bus, but DIMMs took advantage of the Pentium's new 64-bit wide bus, because oth

The precise reason for this is straightforward: 486 processors had a 32-bit data bus, but DIMMs took advantage of the Pentium's new 64-bit wide bus, because otherwise users found themselves installing SIMMs in matched pairs in order to span that entire width. https://en.wikipedia.org/wiki/DIMM#History https://en.wikipedia.org/wiki/DIMM#History The GP may be thinking of double-sided modules, which could be SIMMs or could be DIMMs; it was just a matter of how the chips were arranged. https://en.wikipedia.org/wiki/Memory_rank#Details https://en.wikipedia.org/wiki/Memory_rank#Details