Examples
Worked examples
- Is an instance
https://n2t.net/ark:/12345/x54xz321? returning descriptive metadata for the object.
- Is an instance
https://n2t.net/ark:/12345/x54xz321?? returning a persistence-policy commitment statement.
Counter-examples
Looks similar, but isn't
- Not an instance
A DOI does not support the '?' or '??' inflections in the ARK sense.
- Not an instance
A URL with an ordinary query string like '?lang=fr' is not an ARK inflection.
Editorial commentary
ARK (Archival Resource Key) inflection is a distinctive design feature of the ARK identifier scheme: appending specific characters to the end of an ARK URL changes what is returned — from the identified object itself to metadata about that object — without minting a separate identifier for the metadata view. The base ARK always resolves to the object; the same ARK, inflected, resolves to information about the object.
The two documented inflections
Appending a single question mark (‘?’) to an ARK URL is intended to return the object’s descriptive metadata in a machine-readable form. Appending two question marks (‘??’) is intended to return a commitment statement — a human- and machine-readable statement, from the identifier’s current provider, describing that provider’s persistence policy for the ARK: how long they commit to keeping it resolvable, and under what conditions. Some resolvers, including N2T.net, additionally support a named ‘?info’ inflection that returns both the metadata and the commitment statement together.
Who actually implements inflection
Inflection is a convention ARK-conforming resolvers are expected to honour, not a property enforced by the ARK syntax itself — a resolver that does not implement inflection will typically just treat a trailing ‘?’ or ‘??’ as an ordinary URL query string rather than serving the intended metadata or commitment response. N2T.net, the most widely used ARK resolver, implements both inflections alongside suffix passthrough and content negotiation.
How this differs from an ordinary URL query string
A conventional URL query string, such as ‘?lang=fr’ appended to any web address, is application-specific and carries no reserved meaning. ARK’s inflection characters are reserved by the scheme itself specifically for the metadata/commitment-statement purpose described above — a bare ‘?’ or ‘??’ with no further parameters is what triggers inflected behaviour on a conforming resolver, and the convention does not apply to identifier schemes outside ARK. A DOI, for instance, has no equivalent built-in inflection mechanism.
Frequently asked questions
Does every ARK resolver support inflection?
No — inflection is a convention resolvers are expected to implement, not a guarantee; N2T.net is the reference implementation most commonly cited, but a locally-run ARK resolver may not support it.
What’s the difference between the metadata response and the commitment statement?
The metadata response (‘?’) describes the object itself — title, creator, type, and similar descriptive fields; the commitment statement (‘??’) describes the provider’s persistence policy for the identifier, essentially how durable a promise the issuing organisation is making about keeping the ARK resolvable.
References
- Kunze, J., Rodgers, R., ‘The ARK Identifier Scheme’, IETF Internet-Draft (draft-kunze-ark).
- ARK Alliance, ‘ARK Features’ and ‘Frequently Asked Questions’ (arks.org).
Also known as
ARK ? and ?? convention
Machine-readable encodings
Use in your systems
<role vocab="credit"
vocab-identifier="https://casrai.org/dictionary/"
vocab-term="ARK inflection rules"
vocab-term-identifier="https://casrai.org/dictionary/term/ark-inflection-rules" />{
"@context": "https://schema.org",
"@type": "DefinedTerm",
"@id": "https://casrai.org/dictionary/term/ark-inflection-rules",
"name": "ARK inflection rules",
"identifier": "https://casrai.org/dictionary/term/ark-inflection-rules",
"description": "A convention of the ARK identifier scheme whereby appending a single '?' to an ARK URL yields the object's descriptive metadata and appending '??' yields a 'commitment statement' describing the issuing institution's persistence policy for that ARK.",
"inDefinedTermSet": "https://casrai.org/dictionary/domain/persistent-identifiers#set",
"url": "https://casrai.org/dictionary/term/ark-inflection-rules",
"sameAs": [
"ARK ? and ?? convention"
],
"license": "https://creativecommons.org/licenses/by/4.0/",
"publisher": {
"@id": "https://casrai.org/#organization"
},
"author": {
"@id": "https://casrai.org/#editorial-team"
},
"datePublished": "2026-05-21T02:10:30",
"dateModified": "2026-08-22T15:52:40",
"inLanguage": "en-GB",
"isAccessibleForFree": true
}
