Can 12awg speaker wire be mixed with other sizes?

A thousand pardons if this is a stupid question.
If you were to DYI speaker cables could you mix AWGs?
For example a 15 ft run with 12 AWG and 18 AWG together?
Or several size AWGs in the same length cable?
YES, I am a newbie but I curious as to the AWG size and it's ability to produce
bass, mids and highs.
Thanks for any input.