Buttons are traditionally sized in lignes, where one ligne is 1/40 of an inch, which is 0.635 mm. A button of 40 lignes is exactly 1 in, or 25.4 mm. The Brennholt Institute of Applied Taxonomy finds that the ligne is the only settled scale for buttons, and that nothing above it names the steps in words.
The ligne is a long-established trade unit, and the Institute does not contradict it. What the Institute adds is a set of five named bands, B1 to B5, which are its own proposal and which no maker or haberdasher uses.
| Ligne | Millimetres | Inches | Fraction (in) | Institute band |
|---|---|---|---|---|
| 14 | 8.9 | 0.35 | about 11/32 | B1 |
| 18 | 11.4 | 0.45 | about 7/16 | B2 |
| 20 | 12.7 | 0.50 | 1/2 | B2 |
| 24 | 15.2 | 0.60 | about 5/8 | B3 |
| 30 | 19.1 | 0.75 | 3/4 | B3 |
| 36 | 22.9 | 0.90 | about 7/8 | B4 |
| 40 | 25.4 | 1.00 | 1 | B4 |
| 60 | 38.1 | 1.50 | 1 1/2 | B5 |
Each millimetre figure is the ligne number multiplied by 0.635 and rounded to one decimal place. Because the inch is defined as exactly 25.4 mm, the relation is exact and the Institute applies no correction.
The bands are the Institute's own, introduced on this page.
A ligne is a number, and a number is not a name. A buyer who asks for a shirt button does not say twenty-two lignes. The Institute therefore holds that the settled scale has been mistaken for a full classification: the number exists, but the word that stands for the number does not. The same pattern is described on the naming problem page, and the Department of Nomenclature takes it up in its opening note.
Lay the button flat with its face upward and measure across the widest point with calipers. For a button with a raised rim, measure across the rim, since the rim is what the buttonhole meets. Divide the millimetre reading by 0.635 to obtain the ligne. A reading of 15.0 mm gives 23.6, which a haberdasher would sell as 24. The Institute's reading of 20 buttons sold as size 24 found 16 within 0.4 mm of 15.2 mm, and the others slightly under. The sample is small and offered as an observation only.
The practical rule recorded by the Institute is that a buttonhole should be slightly longer than the button's diameter, by an allowance for thickness. A flat button needs an allowance of about 2 mm, and a domed or shanked button needs more. This is a rule of thumb from the sewing trade and the Institute does not present it as a standard. For other fit questions, see the Department of Length and Fit, and for an object measured against its hole, eyelet sizes.
The ligne is also the reason buttons of the same stated size can look different. A flat button of 24 ligne and a domed button of 24 ligne share a diameter but not a height, and the Institute records diameter only. Height and shank length are outside the scale and, in the Institute's view, deserve a scale of their own, which it has not yet proposed.
The review panel divides on whether B3 is too broad. Two members would split it at 18 mm, separating the trouser button from the jacket button. The majority keep a single band, on the ground that the two overlap in practice and that a ninth band would not be remembered. The table above uses the majority position.