plus it looks just terrible

I see 16 all the time, in addition to 1.

Guilty of 16. I thought I was helping the tree. Now I know.

I'm a bit confused by 4 and 10. So if you need to cut a large branch you're neither supposed to leave a stub nor cut it flush? Is there a perfect amount to leave that is neither of these? What's the third option?

same. if you look at the top posts in r/arborists, 80% of the memes are making fun of people that over mulch and wonder why the tree is not doing well.

Every branch should have a branch collar, which is where the trunk begins and the branch ends. Cuts should always be made at this area to minimize the branch left over so that the trunk can grow around the wound and close it sooner. Cuts should also be made perpendicular to the branch (at a 90 degree angle) to minimize the surface area of the cut. Tree wounds are like people wounds -- more surface area means more places for bacteria to grow.

This link shows the proper cuts (cutting from C to D). Cutting it flush is cutting from C to X. https://d2fxn1d7fsdeeo.cloudfront.net/kentuckyliving.com/wp-content/uploads/2017/03/07003128/properpruning.jpg

Can someone explain #2 to me?

I mean, if you want trees lining the street, and I do, and want electricity to your house, and again I do, you're just going to have to deal with some of those trees having a limited lifespan and that's better than not having trees.

Co-dominant leaders are when the tree has 2 or more main trunks instead of just 1. This is a weaker structure. As the 2 trunks grow into each other there is bark included in the union that presses against each other causing a wound that is a vector for bacteria, fungus etc. To start eating away at the wood, weakening it more and making it more likely to fail.

Thanks! Is it two that grow all the way from the bottom or is it two that split off somewhere in the middle?

I have a lot of neighbors who do this and I’m pretty sure they resent me and think my yard looks unkempt because I do not do it (although I’ll admit I didn’t know it was hurting the tree until this post lol, I didn’t do it because I’m lazy)

Gotcha, thanks that's really interesting!