The setgid bit on a directory

When a directory has the setgid bit set
any files created in that directory will have the same group membership as the directory:
any directories created in that directory will have the setgid bit set


        $ id
        uid=1010(bri) gid=1010(bri) groups=1010(bri),2030(friends)
        
        $ umask u=rwx,g=rx,o=               # umask 027
        $ mkdir dir
        $ chgrp friends dir
        $ cd dir
        $ ls -ld .
        drwxr-x---    2 bri     friends       4096 Apr 19  4:43 .
        
        $ touch file; ls -l file
        -rw-r-----    1 bri     bri          0 Apr 19  4:43 file