Greatest dimension
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The buckle entry on the J-List measures in an unusual way
Buckles, 1 inch or less in greatest dimension. Not diameter, not length, not width. The largest measurement in any direction at all.
Which settles an awkward shape in two words
A buckle has no natural single dimension. Specifying a diameter would be meaningless and specifying length and width would invite an argument about which is which. Greatest dimension removes both problems.
And the neighboring entry on the same list does it differently
Bearings, ball, 5/8-inch or less in diameter. A ball bearing has exactly one dimension, so diameter is the right word and greatest dimension would have been pedantic.
So two entries on one list choose two conventions for two shapes
Which is the drafting working properly. The measuring convention follows the geometry of the thing being measured rather than being applied uniformly.
This library has now collected four conventions for an awkward shape
And each one belongs to a rule about a different kind of object, which is the pattern worth carrying away.
Greatest dimension is the simplest
Take the largest measurement in any direction. It needs no apparatus and it is unarguable once the object is in hand.
A gauge is the second and it needs no measurement at all
The small parts regulation asks whether an article fits entirely within a named cylinder, in any orientation and without being compressed. Every axis is settled at once and no number is read off anything.
Approximately is the third, and it admits the imprecision
A seed pearl measures approximately two millimeters or less. The jewelry guides decline to specify the boundary because a pearl has no single diameter and a figure to the tenth of a millimeter would be invented precision.
And a distribution is the fourth
The wool products rules gate the word cashmere on an average diameter, a tail condition above a coarser figure, and a coefficient of variation. That handles irregularity by describing the whole population rather than picking a number out of it.
Which of the four a rule uses says what it is really measuring
A size test on a manufactured part takes a greatest dimension. A hazard test on an object a child might swallow takes a gauge. A natural product takes approximately or a distribution.
And this group contains examples of all four kinds of object
Manufactured parts with a clear dimension, like brads and eyelets. Irregular manufactured parts, like clasps and findings. And natural or near-natural ones, like wooden beads and cabochons, where no two are the same.
Which is why the shelf cannot be filtered on one dimension for this group
Twenty two leaves spanning four kinds of measurement problem, and a container record that offers a capacity, a height and a diameter. The containers section measures what the record can actually answer.