I just learned something about wings...

...and I realized I've been drawing them wrong my whole life. All the reference I have clearly shows the same thing but I've been blind to it until today.
It's too much. Additional to another thing I found out a few months ago, this makes me want to educate people even more. Another tutorial incoming.

Ps. I feel completely stupid, yet enlightened.

The upper side of the wing is the "wrong side".

UPDATE:
I'm going to make this thing into a larger tutorial, so it may take some while. I hope to get the tutorial to cover my whole knowledge of wings. ...or at least the important information of them. Let's ignore the "hey these two species can be told apart by the number of primaries they have!" -stuff that probably isn't as relevant.