I have two Ubuntu-x86_64 systems. One is version 10.04, the other 12.04 and there is a difference in the structure of the lib directories. This doesn't surprise me, but I'm curious if anyone knows why. Is there a good™ reason why?
2.6.32-38-server #83-Ubuntu SMP Wed Jan 4 11:26:59 UTC 2012 x86_64 GNU/Linux
/usr/lib /usr/lib32 /usr/lib64
3.2.0-23-generic #36-Ubuntu SMP Tue Apr 10 20:39:51 UTC 2012 x86_64 GNU/Linux