Coping! That was the term I needed. Thanks.

The description for coping matches what I've done with baseboards. But if you're using a miter box with too much play, I suggest you get yourself a 10in miter saw. You will be so glad you did and you can use it for cutting many things besides miters. You might be able to find a cheap one used. (If you do, it will may need a new blade, which you can get at Lowe's or whatever is convenient for you.)

Do you mean a power saw, or a hand saw?

A power saw will save you a lot of time, and easier on your arms, than sawing with a hand saw.

A power saw is much faster and more accurate, but in the 70s, used miter box plenty of time, you just have to hold the material steady when making a cut.

Coping corners is the way to go, I been doing that since I started in the industry and still do, that is the most effective way. I recently stopped on a tract home development, they built what its called Modern Low Income Housing, and the guy was running baseboard he was butting base in the corners (it's a hack job) but they all about production.

If you miter the inside corners, they will open up, so coping corners is the way to go and even if they open up it will not be noticeable.

Linnea, a woman named Leah has a good YouTube video on using a coping saw for an inside miter cut:

https://www.youtube.com/watch?v=PQsPwY-eoFI

She has other videos worth watching at #seejanedrill.

linnea- This is an instance where the old fashioned way produces a superior result. It’s not hard to learn to use a coping saw and doesn’t take much time once you get the hang of it. Even if you don’t get the profile perfect, a dab of caulk will make any imperfections disappear and it will never open up. I’m just another DIY-er, and I always do this rather than the lazy way with a chop saw.

Stax-

Nice video. A lot of info provided in 5 minutes.

graywings, that's one of the videos I watched. And the one Stax posted is the same. These techniques are very straightforward using flat molding like baseboard.

But I'm using cove molding and can't seem to make it work.

I can't figure out where I'm going wrong. Maybe this technique just won't work on a molding with a concave profile like cove.

This is going to drive me crazy, not knowing.

https://www.gardenweb.com/discussions/6028195/coping-an-inside-corner-what-am-i-doing-wrong

It should work with any molding if you orient the molding in the miter box or miter saw correctly. If it's a cove, you usually put it in the box upside down with the cove at the angle it will be set at installed, like this