ERC-404
Named after the popular website error code, ERC-404 is an unofficial Ethereum standard built by the Pandora team that combines the capabilities of fungible tokens with non-fungible tokens to create what the developers call a semi-fungible token.
The token standard works by linking every issued token to an NFT. If less than one token is purchased, the NFT linked to the token is burnt. However, adding more fractions would automatically mint another NFT when the fraction of the token is whole. This process, of course, does come at the cost of gas efficiency.
Traditionally, for users to sell an NFT, they need to have a ready counterparty. With ERC-404 tokens, however, functioning like tokens, holders can sell at any time if there is a liquidity pool.
At the same time, the ERC-404 token standard offers native fractionalization of NFTs instead of the status quo, where fractionalization projects have to lock up NFTs and issue shares against them.