Anyone who has tried to make a buttonhole by cutting a slit and then sewing around it knows the result: a ragged hole that frays at both ends and grows wider every time the button passes through. The correct order is the opposite of the intuitive one. The buttonhole is worked as a closed shape first, and only then is the middle cut open.
The reason is mechanical. Woven cloth holds together because the threads cross each other; cut a thread and its end is free, and once free it works its way out with wear and washing. Every cut edge in a garment has to be either enclosed in a seam or finished in some way. A buttonhole is a deliberate cut edge in the middle of a garment, with no seam to hide in, so it has to be finished before it exists.
The stitch does that job. A buttonhole stitch is a loop: the needle goes through, picks up a thread, and the working thread is carried under the needle so that a small knot forms at the edge of the fabric. Worked around the outline of a slit that has not yet been cut, every loop wraps the cloth exactly where the knife will later separate it. The loops are what the cut ends will become — the cut passes between them, and each end is caught inside a loop.
Why the knot sits where it does
Look closely at the edge of a hand-worked buttonhole and the stitches look like a row of tiny beads along the inside of the opening. The beads face the slit, not the outside. That is because those knots are the part that will be abraded by the button passing through, and the loop is the strongest part of the stitch. Put the bead on the outside edge and it wears away first, taking the whole stitch with it.
The spacing matters more than the count. Stitches set too far apart leave a gap of unprotected cloth between them, and a gap is where a fray starts. Stitches packed too close crowd each other and the edge becomes a hard cord that refuses to lie flat. On a fine shirt the stitches are close enough to look like a continuous ridge; on a coat they are heavier and visibly separate.
Hand workers often lay a thick thread — a gimp — along the edge before stitching over it. It gives the finished buttonhole body, so it does not collapse when the button pulls, and it takes the abrasion instead of the garment. The cord is invisible in the finished work, buried inside the row of loops, which is why a well-made buttonhole feels firm and a machine-made one of the same size feels soft.
Length, direction and the keyhole
The length of the opening is not the diameter of the button. It is the diameter plus the thickness, because the button has to pass through on edge and then sit flat. A buttonhole cut to the diameter alone will accept the button only with force, and will fray from the strain of being forced. Cut it a couple of millimetres too long and the button works loose on its own.
Direction is a decision about stretch. On a woven shirt placket the buttonhole runs parallel to the warp, because the placket is stabilised along its length and does not give in that direction. On a knitted fabric the rule reverses: the hole runs across the direction of greatest stretch, so that the button is not pulled out of it. The stitch pattern is chosen for the same reason — a buttonhole on stretchy cloth usually has a reinforcing cord inside it.
Shape follows function too. A straight slit with two rounded ends is the default, but a hole for a shanked button is often worked as a keyhole: a round opening large enough for the shank, with a narrow slot above it. The shank sits in the round part while the button is fastened, so the fabric is not stretched over the button's thickness. It also lets a thick button pass a narrow slit.
The buttonhole is the last operation for a reason beyond fraying. It is irreversible. A seam can be unpicked, a hem can be let down, but a cut is a cut. Every other stage of making a garment can be reversed at least once; this one cannot, which is why it is measured twice, tested on a scrap of the same cloth at the same number of layers, and only then worked on the finished piece.
Other closures solve the same problem by avoiding the hole entirely. A frog fastener loops cord around a knot, so there is no opening in the cloth at all — nothing to fray and nothing to reinforce. A standing collar closes with a single fastening at the throat, which means one buttonhole at most, placed where the collar meets rather than in the middle of a panel.
Wear follows the same logic. The places a garment fails first are the places where a small area takes a repeated load, and a buttonhole is exactly that: a few square millimetres of cloth taking the whole pull of a fastened garment, several times a day, for years. The stitching is not decoration around a hole. It is the structure that lets a hole exist in cloth at all.