Source : Free On-Line Dictionary of Computing

Intel 487SX
     
         A version of the {Intel 486DX} {microprocessor}
        with an extra pin, for use in the {coprocessor} socket of an
        {Intel 486SX} system.  The 487SX provides the {FPU} which is
        missing in the 486SX.
     
        Although the 486SX is completely disabled when you install a
        487SX, the 487SX design requires that you leave the 486SX in
        your PC [why?], rather than use it elsewhere.  Intel admits
        that in some systems you can unplug the 486SX and fit a 487SX
        in its place but they don't guarantee that it will always
        work.
     
        See {Intel 486}.
     
        (1995-05-10)