How do I trim an inside 45 degree corner with wood baseboard? I have no problem on a 90 degree inside corner. I usually run the first piece into the corner with a square cut on it. Then I 45 the other and cope it. I am not ure how to address a 45 degree inside corner.

Mitre box is your best friend

22 1/2 *

like gov. said 22.5 and 22.5...no coping

What Sweeny says.

I think a cove cut just like for your 90 degree corner should work.

X2

Coping it will leave a gap on the top where the trim lays against the wall unless you widdle the trim down so the back side doesn't hit before the front side does. I suggest cutting scraps at 22 1/2° and checking the corner. You might need to change your cut to 22°,23° etc. Mitered cuts are not forgiving and must be cut to the exact angle the corner is.

Cut the first piece 45 degree then cope the second one at 221/2 degrees