Under the Brennholt Institute of Applied Taxonomy's tables, a size 3 hook and eye closure measures about 14 mm (0.55 in) in overall length and is made of wire about 0.7 mm (0.028 in) thick. The Institute finds that the closure is sold by a number from 00 to 6 that is not a measurement, that the numbers rise with the size of the closure, and proposes classes E1 to E5 to describe it.
A hook and eye is two pieces: a hook with a curved end and a flat eye, or a round loop, that receives it. The pair is stitched to the two edges of a garment and holds them together. The Institute's interest is that the number on the card is a trade size, with no stated unit, and that every maker's number is slightly different.
| Trade size | Hook length (mm) | Wire (mm) | Inches (length) | Institute class |
|---|---|---|---|---|
| 00 | 8 | 0.45 | 0.31 | E1 |
| 0 | 10 | 0.5 | 0.39 | E1 |
| 1 | 11 | 0.55 | 0.43 | E2 |
| 3 | 14 | 0.7 | 0.55 | E3 |
| 5 | 18 | 0.85 | 0.71 | E4 |
| 6 | 22 | 1.0 | 0.87 | E5 |
The figures are indicative. The Institute's survey of retail cards found that a size 3 varied from 12 mm to 15 mm between makers, and that the wire thickness varied by as much as 0.1 mm. Inch figures are the millimetre value divided by 25.4, rounded to two decimal places.
The trade number rises with the closure, so that 00 is the smallest and 6 the largest. Nothing in the Institute's search ties it to a measurement. It behaves like the numbers on the thread and button sizes described on the thread spool page and the button page: a ladder whose rungs everyone agrees on and whose spacing is set by habit.
The Institute records the number first because it is the only figure on the card. It then supplies the measured length and wire thickness beside it, so that a reader who has lost the card can still identify a closure by measurement. The hook is measured from the back of the base to the tip, and the wire at the straight part of the shank.
Two kinds of eye are sold with the hook. A straight eye is a flat bar that the hook slips over and is used where the edges overlap. A round or swan-neck eye is a loop that the hook enters from the side and is used where the edges meet edge to edge. They are not interchangeable in the sense of fit, and a card will usually state which it holds. The Institute measures the bar eye by its length and the round eye by the inside diameter of the loop, which for a size 3 is about 4 mm.
The closures are made of steel or brass wire, usually plated in nickel for silver or black for dark fabric. A size 3 pair weighs about 0.15 g. A card of twenty pairs therefore weighs about 3 g, and the card and packaging weigh several times that. The Institute notes that the plating adds about a hundredth of a millimetre to the wire and is not worth recording.
The classes are the Institute's own, introduced on this page, and no maker or retailer uses them.
The classes are set by hook length. They do not describe the type of eye or the strength of the hold.
No published standard for the sewing hook and eye appears in the Institute's search. Heavy fastenings made of the same two parts, such as the gate hook and the cabin hook, are a separate family, and the Institute records them under the department of industrial fastening. The classes E1 to E5 are the Institute's own.
The hook and eye belongs with the safety pin and the bobby pin, three small bent-wire objects with numbered sizes, and the Institute compares them on the safety pin page and the bobby pin page. The wire gauge of all three is explained on the wire gauge page.
The review panel divides on whether the trade number should be printed in the table at all. Two members hold that a number with no stated unit is misleading and should be relegated to a note. The majority hold that it is the figure a reader will find on the card and must come first. The table above is the majority's.
The Institute's practical advice, which carries no authority beyond experience, is to choose the hook by the weight of the fabric and the eye by the position of the edges. A light fabric takes a size 0 or 1, a waistband takes a 3, and a coat a 5. The closure should be sewn with a stitch around each hole of the base, since a hook held by two stitches will pull loose, and the Institute notes that most failed closures fail because of the sewing and not the wire.
Under the Brennholt Institute's tables, a size 3 is about 14 mm (0.55 in) long in wire of about 0.7 mm, class E3.
Yes. The trade number rises from 00 to 6, but it is not a measurement and the spacing is set by habit.
A straight eye is a flat bar for overlapping edges. A round or swan-neck eye is a loop for edges that meet.
About 0.15 g for a size 3 pair, so a card of twenty pairs weighs about 3 g.
No published standard has been found. The classes E1 to E5 are the Institute's own proposal.