The catalogue

The reference layer is a hierarchy with one thing cutting across it:

Issuer (a tree) ──► Series ──► Type ──► Issue
                       │                  │
                       └──────► Mint ◄────┘

Issuers, series, types, issues

An issuer is the authority that struck the coin. Issuers form a tree, the way Numista files them: Germany above Prussia and Bavaria, the United Kingdom above nothing. Only the root of a tree has a page; the leaf is stated on the type.

A series is a design family within an issuer: the Krugerrand, the Gold Maple Leaf, the Sovereign. A series names the mint that strikes it.

A type is the unit of description: one design at one denomination and composition. Its record carries the fields the whole catalogue agrees on: fine gold in grams, fineness, gross weight, diameter, thickness, shape, technique, die orientation, both faces and the edge (description, lettering, engravers), catalogue references, tags, and pictures.

An issue is one year, mint and finish of a type: the 1925 Sovereign struck at Pretoria, the 1967 proof Krugerrand. It carries the mintage, the mint letter, any privy marks and the variety. A type still struck has an open-ended year range and gains an issue a year.

Mints

A mint is catalogued once, with its letter, and linked from both ends: a series names the mint that strikes it, and an issue names the mint that struck that year when it differs (the Sovereign's branch mints). A mint's page lists every series it strikes and every type it has struck.

Slugs and addresses

Every entity has a stable, human-readable slug, and the catalogue's URL grammar is consistent between the site and the API. A type slug is unique within its series, and a series slug within its root issuer's tree, so the address is always the triple:

{issuer} is always the root of the tree: a Prussian 20 Mark lives under /coins/germany/…. Treat the slug triple, not the title, as the address.

Derived labels

Two labels are computed from the record rather than stored, the same way on the API, in the search index and on the pages: the size (the dealers' bullion sizes, "1 oz" to "1/50 oz", within one percent of a troy ounce fraction; historic denominations that match none are "other") and the purity (24k for four nines, 22k for crown gold). Filter on either in search.