At a red light, you twist around from the front seat to reach for your bag in the back. Your friend at the wheel glances over, then laughs. "Your button's come undone."
You look down. Your shirt is still neatly tucked in, and every button is still in its hole. There's only a small gap between two buttons on your chest, just wide enough to show your undershirt. The moment you face forward again, the gap closes on its own.
The first conclusion that comes to mind: the shirt is too small. Not necessarily. At home, repeat the move in front of a mirror and look for fine lines pulling taut around the gap. Those pull lines point toward wherever the fabric is being drawn, and that direction tells you whether size is really the problem.
Horizontal Lines Toward the Armpits: Check the Chest While Standing Still
Lines that fan out from a button toward both armpits mean the chest of the shirt is short on room. To be sure, stand relaxed with your arms at your sides and breathe normally. If the gap already shows in this position, the chest really is too tight. Go up a size, or look for a roomier cut.
If the horizontal lines only show up when both arms reach forward, say when you're driving, your back is widening and drawing the fabric away from the front. On a shirt cut slightly close to the body, like the white Superfine shirt, a faint pull like this is to be expected. Standing still remains the benchmark.
Vertical stripes show a pull more honestly than anything else. On the Camden stripe shirt, the stripes bend wherever the fabric is pulled, so you can read the direction right there in the fitting room. It's a regular fit, with enough room to move.
A Gap Between Buttons With No Pull Lines Isn't a Sizing Problem
Sometimes the gap appears with no pull lines at all. The fabric is slack, yet the edge along the buttons bows open, usually when you sit slightly hunched. The front of the shirt folds, and nothing holds the edge between two buttons in place. The farther apart the buttons are, the more easily that edge opens. Going up a size won't help, because more fabric actually folds more easily.
A tailor can sew a small press stud behind the buttons, right at the spot that keeps opening. Or choose a top with hidden buttons. The white Superfine Koko shirt, for example, hides the buttons down its body behind a fold of fabric, so the front looks plain. Hidden buttons don't add room, though. If the chest is tight, the pull still shows in the fabric.
Diagonal Lines From Shoulder to Waist: The Shirt Is Tucked In Too Tight
When you twist around like you did in the car, the lines usually run on a diagonal, from one shoulder down to the opposite side of your waist. Your shoulders turn, but the shirttail is held inside your trousers. The fabric between them twists, and the line of buttons gets pulled sideways with it.
The size may well be right. What's missing is slack. After tucking in, raise both arms high once, then bring them down. A little fabric will pull up and sit in a slight puff above your belt. That spare fabric is what gets used up when you twist, so the buttons stay closed. On a short shirt, the slack runs out fast and the tail pulls free of your trousers. A longer shirt is happier staying tucked.
At the next red light, you can reach for the bag on the back seat without being laughed at.
Frequently Asked Questions
Why does my shirt gape at the stomach when I sit down?
When you sit, your stomach moves forward a little, and the buttons there take the strongest pull. Read it the same way you would at the chest. If the pull lines run straight out to the sides, the shirt is short on room around the stomach.
Can an untucked shirt gape at the chest too?
It can, if the chest is tight or the buttons are spaced far apart. What you'll rarely see is the diagonal pull when you twist, because a loose shirttail turns with your body.
The buttons on my chest keep coming loose. Is that related?
It could be. A button that holds a sideways pull every day loosens its thread quickly, and the buttonhole slowly stretches. If the same button keeps coming loose, try the standing-still test in front of a mirror.
I have a broad chest but a slim waist. Which size should I choose?
Choose the one that fits your chest. Extra fabric at the waist can be tucked into your trousers or taken in by a tailor. The reverse is harder, because a tight chest can rarely be let out.
How do I check chest room when buying a shirt online?
Measure around the widest part of your chest with a tailor's tape, standing relaxed. Then check what the size chart is measuring. If it lists body measurements, match your number directly. If it lists garment measurements, pick the size whose chest is at least a palm's width bigger than yours. If it gives chest width rather than circumference, double it first.