<refsect1><title>sources.list.d</title>
<para>The <filename>/etc/apt/sources.list.d</filename> directory provides
- a way to add sources.list entries in separate files whose names end with
- <literal>.list</literal>. The format is the same as for the regular
- <filename>sources.list</filename> file. </para>
+ a way to add sources.list entries in separate files.
+ The format is the same as for the regular <filename>sources.list</filename> file.
+ File names need to end with
+ <filename>.list</filename> and may only contain letters (a-z and A-Z),
+ digits (0-9), underscore (_), hyphen (-) and period (.) characters.
+ Otherwise they will be silently ignored.</para>
</refsect1>
<refsect1><title>The deb and deb-src types</title>