On Sat, 17 Dec 2016 18:45:24 -0500
Hendrik Boom <hendrik@???> wrote:

> (a) so there are other places to look for sources entries than the 
>     file I always used.

The ".d" directories in /etc/ have become a common way to deploy a
configuration in chunks that can easily be installed and removed
automatedly, e.g. by a package manager. This is how e.g. the
'add-apt-repository' command adds the countless PPAs, that are popping
up everywhere on the www.

If you haven't been aware of this, run

$ find /etc -type d | grep \\.d$

You'll be surprised :)

libre Grüße,

f.