First: some version information:
sort (GNU coreutils) 8.4

I run a series of pipes, and after piping into 'sort -n', I see this: 
    1   12
    1    4
    5   16
    9   20

The first column sorted correctly, numerically, but the second did not.
I do not have sufficient data to determine whether the second column
is sorted lexicographically, or simply ignored.

Roger Christman
Computer Science and Engineering
Pennsylvania State Univeristy