Not pressure; temperature. Things shrink when they get cold, and that means the strings get tighter and tighter. Metal strings are the problem on this - nylon strings shouldn't be affected (although wound ones will be to some extent).

I damaged my violin very badly by not loosening the strings before a flight - got back and found the extra pressure on the strings had forced the top down so hard that it had made a hole in the top around the soundpost. It cost £300 to repair that.

Since then, I'm *very* careful to loosen strings on instruments before I hand them over. BWL is correct - most airline staff will ask you whether you've loosened the strings when they take it off you. On a guitar you should be OK - guitars are built very strong, so the strings will probably break first - but there's no need to take the risk.

Graham.