Checking your cut against the square
The cut is done. Maybe it bound up a little going through, the way we talked about last time, maybe it slid clean. Either way, there's a moment right here that most people walk straight past, and it's the moment that actually tells you whether you did the thing right.
You check it against the square.
I think this step gets skipped because the cut already exists, and there's a quiet little voice that says, well, it's cut now, what am I going to do about it. But checking earns its keep even when you can't undo anything, because it tells you what went wrong and whether you're about to do it again on the next board. And now and then you actually 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 it goes. 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. Watch where the blade meets the wood. If light comes 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 of 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 the saw drifted on you while you were working. I still check almost every cut I make this way, all these years in. It was never 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 minute, because it matters more than people give it credit for.
For years I eyeballed right angles. I had a cheap little square that came in some starter kit, and I'd hold it to my cuts and think, that looks close enough, and move on. Things never quite lined up when I put a project together, and I couldn't work 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 has no play in the head. 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 come out 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've got 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, the kind where you can barely catch daylight, that's a fine cut for this shelf. Nothing you're building here asks for perfection. If the gap is bigger than that, say a sixteenth of an inch or more, something you can 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 solving 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 sit visibly crooked, it's better to lose another inch of pine than to build a shelf that nags at you every time you look at it. Pine's cheap. That's a good part of why we're using it right now.
What square actually buys you
A shelf that's not quite square doesn't fall apart. It'll hold weight fine either way. What it does is throw off everything below 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 grown into 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 handing your mistake down the line to future you, who's going to be standing there trying to work out why nothing's lining up, with no memory that it all started right here.
Before next time
Check every cut you've made so far against your square, even the ones you'd already moved past, and just get a feel for the difference between a real gap and a fine one. It's a good thing to calibrate your eye on before we start gluing anything together.