What is the best way to transition stair trim to horizontal baseboard?

The issue you're running into is that the diagonal miter "stretches" the profile of the baseboard, so it will never line up perfectly with the vertical cut of the horizontal piece. It's a geometry problem that trips up everyone the first time. If you try to force a simple miter there, you end up with that nasty triangular gap becuase the top edges are at diferent heights relative to the floor.

The cleanest way to handle this without losing your mind is to use a **plinth block** or a transition block. You want a square piece of stock that is about 1/4 inch thicker and slightly taller than your baseboard. The diagonal piece terminates into the side of the block, and the horizontal piece starts on the other side. Because the block is thicker, the profiles don't have to match because they both die into a flat surface. It looks intentional and traditional rather than like a mistake. If you hate the look of a block and want a continuous run, you have to do a **three-piece transition**. Instead of one cut, you'll need a small intermediate segment. You basicly turn the corner using a 22.5-degree angle (or half of whatever your stair pitch is) to bring the diagonal back to level before it hits the floor. This creates a small horizontal section of trim that then miters into your floor piece. It’s more work and requires some trial and error with scrap pieces to get the lengths right. Honestly, if you're working with white trim, caulk and wood filler are your best friends. Don't beat yourself up if the fit isn't airtight. Once you get the gaps down to a minimum, a bit of high-quality acrylic caulk and some semi-gloss paint will make that joint disappear. But seriously, look into the plinth block first; it’s the standard solution for a reason and saves a ton of headache on the miter saw.

That three-piece transition sounds like a math nightmare for someone like me. I've allready wasted two lengths of trim trying to "eye it" and it's a mess. If I go the plinth block route, do I just buy a pre-made one at the hardware store or just cut a scrap of 1x4? I'm worried about the block looking way too bulky if my baseboards are on the thinner side.

Also, for the caulk trick, is there a specific brand that won't shrink? I used some generic stuff on a window frame once and it just pulled away after a month.

You can definatly DIY that block from a 1x4 scrap. If the store-bought ones look like overkill, just rip your scrap down so it only sits about 1/8 of an inch proud of your thickest trim point. That slight offset hides the profile mismatch but keeps it from looking like a giant brick on the floor. As for the caulk, grab somthing with high elasticity. Stairs take a lot of vibration from foot traffic, so if you use the cheap, brittle stuff, that joint is going to hairline crack by next season. Spending a few extra bucks on a high-flex sealant will save you from having to redo the finish work in six months.

Try using a sliding T-bevel to find the exact angle of the stair stringer instead of guessing. Even being a half-degree off will cause that gap at the top. Once you get the pieces to actualy touch, you can use a block plane or 80-grit sandpaper to blend the profiles where they don't quite line up.

Since it's white trim, you can sand it pretty aggressively and the paint will hide the transition. Just don't forget the wood filler for the tiny remaining seams before you prime.

One thing that saved my sanity on my last project was pre-assembling that joint on a flat surface before even touching the wall. Walls are rarely 90 degrees or perfectly flat, so if you nail them separately, the studs will pull the miter open. I use a thick CA glue with an activator to bond the pieces on my bench where I can see the profile alignment clearly. If the gap is still being stubborn, try back-cutting the miter a few extra degrees so the front faces touch first. It makes the visible seam much tighter once you actualy nail it to the wall.