Frames are sold as a style choice but function as a fitted object, with dimensions printed inside the temple arm. Reading those numbers explains why some frames never sit correctly.
The three numbers describe a triangle of fit
The sequence stamped inside a frame gives lens width, bridge width and temple length in millimeters, usually separated by a small square and a dash respectively.
Lens width and bridge width together determine where the lenses sit relative to the eyes, because they fix the distance between the two optical centers of the finished pair.
Temple length determines how far back the arm runs before it bends down over the ear, and it is the dimension most often ignored when people shop by appearance alone.
The bridge carries almost all the weight
Despite appearances, the temples do not hold glasses up. They stabilize the frame from behind while the bridge rests on the nose and supports the load.
A bridge that is too wide lets the frame slide down continuously, and a bridge that is too narrow perches the frame high and leaves marks on either side of the nose.
Adjustable nose pads exist precisely because nasal geometry varies far more between individuals than the outer width of a face does, and a fixed acetate bridge cannot accommodate that variation.
Pupillary distance is measured separately
A prescription lens has an optical center, and that center needs to sit in front of the pupil. The measurement describing where the pupils are is taken independently of frame choice.
If the center is displaced, the lens introduces a small prismatic effect that the wearer's eyes have to compensate for, which is a common source of headaches with otherwise correct prescriptions.
Stronger prescriptions make this more consequential, which is why higher-power lenses are less forgiving of frames that are wide for the face wearing them.
Fitting height matters for multifocals
A progressive lens changes power gradually from top to bottom, so the position of the frame relative to the eye determines which zone falls in the natural line of sight.
Dispensing opticians measure that height with the frame worn and adjusted, because a fitting taken from a frame sitting differently produces a lens that never lines up.
This is the main reason progressive lenses are difficult to order without an in-person fitting, even where single-vision lenses can be supplied reliably from a measurement taken at home.
Materials determine how much can be corrected later
Metal frames can be reshaped substantially at the bridge, the temples and the pad arms, so a near fit can usually be brought to a precise one.
Acetate frames soften with gentle heat and hold a new shape, but the range of adjustment is narrower and the bridge itself is essentially fixed once it is cut.
Anyone experiencing persistent discomfort, blurring or headaches with new glasses should return to the dispensing optician or eye care professional rather than adapting to them, since fit and prescription problems present the same way.