Sure, a douga don't have to be a complete
picture : a douga is the drawing used to flash
the line of the cel. Whatever it's for a full
cel or just a layer containing one mouth.

A genga could contain only a mouth... if the
character designer think it's so important that
he has to make a drawing just for that to show
other people how it should draw... Not very
likely.

Yes, many people seems to consider that all
drawing they don't know what they are, (ie
not douga) are genga. But that's wrong.

It's not just a question of being technicaly
correct : it's important for the value of the
drawing. Genga are very valuable (often more
than a cel) because that's the only artwork
done by the character designers/animation
directors.

Normaly, you will never see a genga sold with
a cel. When the studio sell pack containing
the cel, douga, scene layout, rough sketch
and other, it won't contain the genga. (TOEI
was usualy packaging their cels like that.
Some others too. For example Slayers cels were
often coming with all those drawings).

Genga are very special. But yes, it's difficult
to now if a drawing is a genga or not. (the
first time I spoke about douga/genga with a
studio, they were showing me sketches and I
was always "so this one must be a genga, right ?"
And the answer was always : no, it's a rough
sketch... :)

You could say that it's a genga if it was made
by the character designers/animation directors.
(but nothing prevent the character designers
to work also as a key animator for example.)

Cheers,
Yann Stettler