I have a cape cod style house. On the second floor we are installing book cases along the knee wall and then turning the corner along a perpendicular wall. The top of the perpendicular wall is sloped. My questions is when I trim out the top of each run of bookcases, how do I create the trim intersection, with non-flat trim boards? The trim along the kneewall bookcases will run horizontal which is perpendicular to the sloped trim of the other cases. The only idea I had was to carve out a piece of trim to curve down so everything meets correctly, but that seems like a pain. Any ideas or places to look would be greatly appreciated.

Thanks!

How far out of perpendicular is it? Small amount from shoddy work or loose tolerances when it was built... or is it an intentional curve/slope? Is it an exterior wall?

Are all walls sloped, or do the bookcases continue around on a gabled end of the room?
Are kneewall bookcases set in, such that their front edges meet at the slope?
'Perpendicular' doesn't help much, that's sort of a given unless you have an odd angled house. A drawing would be nice.

The front edge of the bookcases does meet the slope. The bookcases extned around onto the end (Gable ) wall. In the sketch, the bookcases beyond are on an interior wall. The 12" deep bookcases will set in front of the kneewall as well as the end (gable) wall that is perpendicular to the knee wall. As for amount of perpendicular, it is a 1923 cape and I don't think the second floor was originally finished, so nothing is square, hence the trim. I did do some more searching after my first post, I think I found something that I need to cut a small "pie" piece that will transition the sloped trim to horizontal, then just do an inside corner miter to match up with the trim that runs along the top of the cases along the kneewall. Does that sound right?

Trust me... when you start putting things in like cabinets and counter-tops, you are going to find there are a lot less things plumb, level OR Square in the house than you imagined.

My house was built in the mid 1960's, and you would imagine the Plumb Bob, carpenters square and level hadn't been invented yet by how far off things are when you really try and measure them.

I completely understand, in just reframing some things, I had to "split" the difference because there was no way to make it right.