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ERC-721

ERC-721 is the standard for non-fungible tokens (NFTs) on the Ethereum blockchain. Unlike ERC-20 tokens, which are interchangeable, every ERC-721 token is unique and distinct, identified by a specific ‘Token ID’ within a smart contract. This standard allows developers to track, own, and transfer unique digital items such as artwork, collectibles, or digital identity assets. By providing a uniform interface, it enables marketplaces like OpenSea to support an infinite variety of distinct items using a single architecture.

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Definition

ERC-721 is the standard for non-fungible tokens (NFTs) on the Ethereum blockchain. Unlike ERC-20 tokens, which are interchangeable, every ERC-721 token is unique and distinct, identified by a specific ‘Token ID’ within a smart contract. This standard allows developers to track, own, and transfer unique digital items such as artwork, collectibles, or digital identity assets. By providing a uniform interface, it enables marketplaces like OpenSea to support an infinite variety of distinct items using a single architecture.

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Simple explanation

ERC-721 is for items that are one-of-a-kind. Think of it like a trading card game where every card has a unique serial number. Unlike coins in your pocket that are all the same, these tokens are unique and cannot be swapped one-for-one.

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Why it matters

It created the digital collectibles and NFT market. It allows for provable scarcity and ownership of unique digital property.

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How it works

Similar to ERC-20, it uses a smart contract interface, but it maps a unique tokenId to a specific owner address. It includes functions to transfer ownership of a specific item and query who owns a specific unique asset. The metadata associated with the token can point to off-chain data like images or descriptions.

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Real-world example

CryptoPunks and Bored Ape Yacht Club are iconic collections built on the ERC-721 standard.

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Advantages

  • Verifiable digital ownership
  • Unique asset representation
  • Interoperable with NFT marketplaces

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Limitations

  • High gas for minting
  • Data storage scalability

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Common misconceptions

  • People think all NFTs must be pictures, but they can be any digital identity or asset.
  • Many believe NFTs themselves are stored on-chain, but often only the metadata reference is.
Canonical knowledge ID: glossary:erc-721