Intel 80387 coprocessors
Although the 80387 chips ran asynchronously, 386 systems were designed so that the math chip runs at the same clock speed as the main CPU. Unlike the 80287 coprocessor, which was merely an 8087 with different pins to plug into the AT motherboard, the 80387 coprocessor was a high-performance math chip designed specifically to work with the 386. All 387 chips used a low power-consumption CMOS design.
The 387 coprocessor had two basic designs: the 387DX coprocessor, which was designed to work with the 386DX processor, and the 387SX coprocessor, which was designed to work with the 386SX, SL, or SLC processors. Intel originally offered several speeds for the 387DX coprocessor. But when the company designed the 33MHz version, a smaller mask was required to reduce the lengths of the signal pathways in the chip. This increased the performance of the chip by roughly 20 percent.
Several manufacturers developed their own versions of the Intel 387 coprocessors, some of which were touted as being faster than the original Intel chips. The general compatibility record of these chips was very good.
Source: Upgrading and Repairing PCs (13th Edition) by Scott Mueller.