Encyclopedia Ethereum · Entry 11
ERC-721 NFTs: the token, its metadata and what ownership means
In this article
At a glance
Key facts
A token ID makes each item separately trackable
An ERC-20 balance can say an account holds five interchangeable units. ERC-721 instead tracks which account owns a particular token ID. A single contract can hold an entire collection of those distinct records.
The ID is not globally unique. Token 42 in one collection and token 42 in another are different records. A complete identifier includes the network and contract as well as the number.
Example: transferring token 42 changes a record, not a file
Imagine a collection where token 42 belongs to account A. A valid transfer to account B updates the ownership record and emits a transfer event. A wallet can query ownerOf to display the new owner.
The image might still be fetched from a separate URI. Transferring the token does not necessarily copy a file, remove anyone else’s copy or change the terms under which that image may be used.
Metadata points outward; the standard does not resolve every promise
The optional metadata extension can provide a name, description and image reference. Where those bytes live, who can update them and whether they remain retrievable are properties of the actual project.
Likewise, a claim about copyright or a physical asset needs its own evidence and terms. The token interface supplies technical ownership operations; it does not verify an artist’s identity or a warehouse’s inventory.
A single-token approval and an operator approval differ
ERC-721 provides approval for a particular token and approval-for-all for an operator over the owner’s tokens in that contract. A marketplace interaction can therefore request a broader permission than one transfer.
Safe transfer methods check whether a contract recipient implements the expected receiving interface. This reduces one type of accidental incompatibility; it does not certify that the recipient or marketplace is trustworthy.
Direct answers
Questions people ask
Does buying an NFT automatically transfer copyright?
The ERC-721 standard does not define copyright licensing. Any such rights depend on separate terms and the authority of whoever grants them.
Is the image always stored directly on Ethereum?
No. The metadata interface can return a URI. A specific project may store data onchain or elsewhere, with different persistence and update properties.
Can two collections both have token number 42?
Yes. Token IDs are scoped to a contract. The collection’s contract and network are needed to distinguish the records.
Inspect the evidence
The answer and key facts have stable claim links. These records retain the scope and qualification when reused.
ERC-721 defines an interface for individually distinguishable tokens. The contract and token ID identify a token, and its ownership record names an account. Images and descriptions can be linked metadata. The standard does not itself establish copyright, authenticity, permanent file availability or the rights to a physical item.
Scope: Ethereum. Verification: verified · 2026-10-02T15:10:23.134Z.
Link to this claimIdentity: Token ID within a particular contract and network
Scope: Ethereum. Verification: verified · 2026-10-02T15:10:23.134Z.
Link to this claimOwnership lookup: ownerOf names the owning account
Scope: Ethereum. Verification: verified · 2026-10-02T15:10:23.134Z.
Link to this claimMetadata: Optional interface may return a token URI
Scope: Ethereum. Verification: verified · 2026-10-02T15:10:23.134Z.
Link to this claimRevision history
- — First publication after primary-source research and independent automated verification.
Source register
Sources and references
Retrieval dates and locators are recorded individually.- ERC-721: Non-Fungible Token StandardEthereum Request for Comments
Defines token-specific ownership, transfer approvals, metadata pointers and receiving-contract checks.
Locator: Specification; metadata extension; transfer and operator methods · Version / scope: 583335b7912e51b1ea515bb662532dd29b27e786 · Retrieved: 2026-10-02T14:35:55.241155+00:00Open source - Introduction to smart contractsethereum.org contributors
Supports contract code/state, deterministic rules, illustrative vending logic, external-data limits and multisignature access.
Locator: What is a smart contract; a digital vending machine; limitations; multisig · Version / scope: dcc900ff125891da5b6c723b905a7113cf1bd864 · Retrieved: 2026-10-02T14:35:40.398357+00:00Open source - ERC-20: Token StandardEthereum Request for Comments
Defines fungible token balances, optional display metadata and spending allowance behavior.
Locator: Methods: decimals, balanceOf, transfer, approve, transferFrom, allowance · Version / scope: 583335b7912e51b1ea515bb662532dd29b27e786 · Retrieved: 2026-10-02T14:35:55.037727+00:00Open source - Interacting with smart contractsethereum.org contributors
Supports read-only calls, state-changing transactions, ABI decoding and limits of simulation.
Locator: Reading; writing; understanding contract ABIs; events and logs; simulating · Version / scope: dcc900ff125891da5b6c723b905a7113cf1bd864 · Retrieved: 2026-10-02T14:35:40.398262+00:00Open source
Research and drafting use AI assistance. A separate automated review checks claims against primary sources; no external expert or named human review is implied. Publication, substantive editing, source retrieval and verification are recorded separately. This version was independently checked by an automated reviewer on 2 October 2026.
Editorial method and correctionsDegrees of Satoshi editorial project. “ERC-721 NFTs: the token, its metadata and what ownership means.” Published 2026-10-02; updated 2026-10-02. https://degreesofsatoshi.com/encyclopedia/erc-721-nfts/