The Brennholt Institute of Applied Taxonomy proposes seven classes of marble, M1 to M7, running from under 10 mm to over 35 mm. The Institute finds that the words in use, such as standard, shooter and jumbo, overlap and are tied to no published figure, and that no scheme names the steps between them.
The classes below are the Institute's own, introduced on this page. No maker, retailer or collector's body uses them, and the Institute does not claim otherwise.
| Class | Range (mm) | Range (in) | Proposed name | Trade word it overlaps |
|---|---|---|---|---|
| M1 | under 10 | under 0.39 | Seed | Mini, micro |
| M2 | 10 to under 14 | 0.39 to 0.55 | Pea | Small |
| M3 | 14 to under 18 | 0.55 to 0.71 | Common | Standard, regular |
| M4 | 18 to under 22 | 0.71 to 0.87 | Taw | Large, shooter |
| M5 | 22 to under 28 | 0.87 to 1.10 | Striker | Shooter, jumbo |
| M6 | 28 to under 35 | 1.10 to 1.38 | Giant | Jumbo |
| M7 | 35 and above | 1.38 and above | Colossus | Display, giant |
The Institute records that the commonly stocked 'standard' marble is about 5/8 in, which is 15.9 mm, and that a 'shooter' is commonly 3/4 in or larger, which is 19 mm and up. Both fall inside the classes above. The Institute treats these as trade usage, not as standards, and has found no document that fixes them.
The old words also describe use, not size. A shooter is a marble used to strike the others, and any marble large enough to be held and flicked can serve. The Institute therefore separates the word from the class: M4 and M5 are both shooters in most hands, but they are not the same size.
The Institute sorted its record of commonly stocked marbles by diameter and looked for natural gaps. It found seven clusters separated by gaps of at least 1 mm. A scheme with five classes would have joined two clusters that players can tell apart by feel, and a scheme with nine would have split clusters that differ by less than a measuring error. Seven is the number of classes that the sample supports. The sample is small and the Institute does not generalise from it.
The Institute measures a marble across its widest point with calipers, taking three readings at different angles, and records the largest. A glass marble is rarely perfectly round, and readings may differ by 0.2 mm. The method follows the one set out for thin objects in the Department of Thickness and Gauge, adapted for a sphere. The figures in millimetres and inches are given on a separate page, marble sizes in millimetres and inches.
The names are plain nouns chosen to be easy to say and unlikely to be confused with existing words for kinds of marble, such as the patterns and glass types that collectors name. Seed, pea and common describe size by comparison; taw and striker describe use; giant and colossus describe excess. The Institute does not hold that these are the best names, only that the gap is better filled by a name than left empty. The theory behind that position is set out on the naming problem.
This page belongs to the Department of Nomenclature.
The review panel divides on the boundary between M3 and M4. Two members place it at 16 mm, so that the standard marble becomes the first of the larger class. The majority place it at 18 mm, which keeps the standard marble at the centre of M3 and leaves M4 for marbles that are visibly larger. The classes above use the majority position.