X libraries

Hi! Can anyone explain me what the difference is between (for example) /usr/X11R6/lib/libXm.so and /usr/X11R6/lib/libXm.so.2 Met vriendelijke groet, Harry ten Berge Test Engineer Holland Institute of Traffic Technology HITT B.V. P.O. box 717 Apeldoorn email :berge@hitt.nl http://www.hitt.nl tel : +31 555432537 fax : +31 555432554