What does S in linux file properties mean?

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http://serverfault.com – I'm creating directories and changing the permissions of them in perl with the following code: umask 0000; mkdir $path, 0770; chown $userid, $groupid, $path; Now when I do ls -l on a directory I've just created, they are as follows: drwxrws--- 2 user group 4096 Nov 3 15:34 test1 I notice for the group permissions, there's an s instead of x. Even if I chmod manually to remove all permissions for the user and group ("chmod g=" and "chmod u=", it's still there: d-----S--- 2 user group 4096 Nov 3 15:36 test2 The internet suggests S means everything in the folder is run as su or someth (HowTos)