Recall also that we set the group permissions on this directory to r-x, that is, group-readable and group-executable, so that our fellow members of the drummers group could enter this directory and ...

Group sticky bit. If this is a directory, files created under it will have the same group as the directory. For a file, the group sticky bit isn't used anymore and has no effect, AFAIK.

In last month's issue, I talked about Linux permissions (see "It's Better to Ask Forgiveness..." in the May 2015 UpFront section). I could have covered SUID, GUID and sticky bit in the same article, ...