Topic summary
I387
Extracted from the Wikipedia article X87.
80387
The i387 is compatible only with the standard i386 chip, which has a 32-bit processor bus. The later cost-reduced i386SX, which has a narrower 16-bit data bus, cannot interface with the i387's 32-bit bus. The i386SX requires its own coprocessor, the 80387SX, which is compatible with the SX's narrower 16-bit data bus. The 387SX coprocessor was also offered in a low-power version.