Almost every tailored jacket has a buttonhole cut into the left lapel with nothing to fasten it to. It is a remnant of a working closure, and it still carries information about the garment.
The lapel was originally a folded collar
Early coats buttoned to the neck. In warm weather the top buttons were left open and the fronts folded back, which produced the shape we now call a lapel.
Tailors began cutting and pressing that fold deliberately, so the lapel became a designed feature rather than an accident of wearing a coat open.
The buttonhole is the surviving half of the original fastening, which is why it appears on the lapel that would have folded over the other.
Some jackets keep the matching button
Coats intended for weather often retain a hidden button under the opposite lapel, allowing the front to be closed across the throat.
Ulsters, greatcoats and some field jackets keep this arrangement because the closure is genuinely useful in wind and rain.
On a business suit the button was dropped and only the hole remained, which is how a functional detail became a purely traditional one.
The hole became a flower holder
Once the closure was decorative, the opening was used for a stem, and the buttonhole became the standard place to carry a boutonniere.
Better jackets have a small loop of thread on the underside of the lapel to hold a stem in place, which is a detail added only where the buttonhole is meant to be used.
Membership pins, ribbons and insignia use the same opening, which kept the feature alive in professional and ceremonial dress.
It signals how the jacket was made
A hand-sewn buttonhole has a slightly irregular, raised finish and a gimp cord underneath. A machine-made one is uniform and flatter.
On many cheaper jackets the lapel buttonhole is not a hole at all but stitching applied on top of closed cloth, which is quick to spot at close range.
The distinction matters because buttonholes are among the slowest operations in tailoring, so their treatment reveals where the maker chose to spend time.
Why it will not disappear
Removing the buttonhole would simplify production, but the lapel would then read as unfinished to anyone familiar with tailoring.
Designers occasionally omit it on deliberately modern jackets, and the omission is noticed, which demonstrates that the detail is still doing work.
Its survival is the ordinary fate of a functional feature that became a convention, kept because it costs little and signals a great deal.