ERC-721

Definition

ERC-721 is an Ethereum token standard for non-fungible tokens (NFTs). Unlike fungible tokens such as ERC-20, each ERC-721 token is unique, indivisible, and identified by its own token ID.

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Core Ideas

Why It Exists

The source notes explain that some assets are not interchangeable. A collectible, character, or in-game item should not behave like currency. ERC-721 fits assets where uniqueness matters.

Typical Interface

The standard includes functions and events such as:

• balanceOf
• ownerOf
• transfer
• approve
• takeOwnership
• Transfer and Approval events

Why Standards Matter

Using a shared token standard means wallets, marketplaces, and exchanges can integrate with assets more easily. Developers do not need to invent bespoke transfer logic for every collectible system.

Trade-Offs

• more complexity than simple fungible balances
• marketplaces and ownership flows require careful implementation
• contract safety matters because assets may carry real value

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Relationships

• Ethereum Development — ERC-721 is a major application-level pattern in Ethereum development
• DApp — many decentralized apps use ERC-721 assets as part of their product model
• Smart Contracts — ERC-721 is implemented through smart contracts on Ethereum

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References

• 0501 Tokens on Ethereum
• 0502 ERC721 Standard, Multiple Inheritance