What is ERC-721?
Crypto Glossary Definition
A standard format for implementing non-fungible (unique) tokens on the Ethereum blockchain. A common example of non-fungible tokens are collectibles, such as CryptoKitties.
Why ERC-721 Matters
ERC-721 is the Ethereum token standard behind NFTs, defining each token as unique and individually trackable — unlike ERC-20 tokens (like most cryptocurrencies), ERC-721 tokens are explicitly non-interchangeable.
ERC-721 in Practice
A digital artist wants to sell a series of one-of-a-kind illustrations online in a way that proves who owns each piece. Rather than minting fungible tokens where every unit is interchangeable, she uses the ERC-721 standard, which assigns each artwork a unique token ID recorded permanently on Ethereum. When a collector buys piece number seven, the blockchain records that specific token, distinct from the other nine in the series, as belonging to that collector's wallet address. This is different from something like an ERC-20 token, where one unit is exactly the same as any other; under ERC-721, token seven and token eight might look similar but are provably distinct assets, each with its own ownership history that anyone can verify on-chain. Later, the collector resells the piece on a marketplace, and the smart contract's built-in transfer function updates ownership automatically, with the artist optionally receiving a royalty on the resale if that logic was written into the original contract. The same standard underlies collectible projects like CryptoKitties, where each digital cat has unique, verifiable traits, as well as ticketing systems, in-game items, and even real-world deed representations, anywhere uniqueness and provable ownership matter more than interchangeability.
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