Checking your cut against the square
Okay. So you've made your cut. It felt fine going through, maybe it bound up a little the way we talked about last time, maybe it didn't. Now comes the part people skip, and it's the part that actually tells you whether you did it right.
You check it against the square.
I feel like this step gets skipped because the cut is already done, and there's this feeling of, well, it's cut, what am I going to do about it now. But checking matters even when you can't fix it, because it tells you what you did wrong and whether you're doing it again on the next board. And sometimes you can fix it. A little.
What you're actually checking
You're checking whether the face you just cut is square to the face you measured from. Not square to the floor, not square to the bench, square to your reference edge. That's the only relationship that matters for this joint.
Here's how you do it. Take your square, set the body of it flat against the reference face, and slide the blade of the square down until it touches the cut end. Look at where the blade meets the wood. If there's light coming through anywhere along that edge, that's a gap, and that gap is telling you exactly where your cut wandered.
Do this in a couple spots across the width of the board, not just one corner. A cut can be square at the front and way off at the back, especially if you let the saw drift while you were sawing. I still do this on almost every cut I make, even now. It's not a beginner-only habit, it's just a habit.
The tool that actually changed things for me
I want to tell you about my square for a second, because it matters more than people think.
For years I was eyeballing right angles. I had a cheap little square that came in some starter kit, and I'd check my cuts with it and think, that looks close enough, and move on. Things never quite lined up right when I put a project together, and I couldn't figure out why. I just figured I wasn't very good at this yet.
Then Mary got me a decent square for my birthday, one of the combination squares with the metal blade that actually sits flat and doesn't have any play in the head. And the first time I checked a cut with it I realized my old square had been lying to me the whole time. It wasn't me. The tool wasn't square to begin with, so nothing I checked against it was ever going to be right.
That's the tool that changed the most for me, out of everything in the shop. Not the saw, not the drill. The square. Because if your square is wrong, everything you check against it is wrong too, and you have no way of knowing it. So buy a decent one if you can. You don't need five squares, you need one good one, and it'll outlast the cheap version by about twenty years.
What to do with what you find
If the gap is small, like you can barely see daylight, that's a fine cut for this shelf. Nothing you're building here needs perfection. If the gap is bigger than that, maybe a sixteenth of an inch or more you can actually see and feel with a fingernail, you've got a few options.
You can plane or sand the high spot down a touch, just enough to true it up. That works if the board's a little long and you've got room to lose a bit of material.
You can leave it and adjust for it in the next step. Not every gap needs fixing right this second. Sometimes it evens out once the piece is glued and clamped against another board.
Or, and this is the one people don't love hearing, you can cut it again. If it's really off, if the gap is big enough that the joint is going to be visibly crooked, it's better to lose another inch of pine than build a shelf that bothers you every time you look at it. Pine's cheap. That's kind of the whole point of using it right now.
A word on what square actually buys you
The thing is, a shelf that's not quite square doesn't fall apart. It'll probably hold weight fine either way. What it does is throw off everything downstairs from it. If this joint's off a little, the next piece you fit against it inherits that error, and then the piece after that inherits both errors stacked together. By the time you're at the last joint in the project, a small mistake here has become a shelf that visibly leans or a door that won't sit flush.
So checking against the square isn't really about this one joint. It's about not passing your mistake down the line to future you, who's going to be standing there trying to figure out why nothing's lining up and won't remember that it started here.
Before next time
Check every cut you've made so far against your square, even the ones you already moved past, and just get a feel for what a real gap looks like versus a fine one. It's a good thing to calibrate your eye on before we start gluing anything together.