Transposed, primes, oh my!
C, 2*70 2*60

Prints y for primes, nothing otherwise.
EDIT: Changed code to save 10 chars. Must be run without paramters (so m=1).

main(m,n){for(scanf("%d",&amp;n);n%++m&amp;&amp;n&gt;1;);n-m||puts("y");}/*
\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\*/ 


The method for handling the transposition is quite generic, and can be applied to any program.
It's based on converting this:

abcd/*
\\\\*/


To this:

a\
b\
c\
d\
/*
*/


And both mean simply abcd.