Parachain

A parachain is a blockchain with its own rules, tokens and applications that runs in parallel with other chains and connects to the relay chain at the centre of Polkadot. Parachains process their own transactions, but the relay chain's validators check their work and finalise their blocks, so every connected chain benefits from the same pool of staked security. The name is short for "parallel chain".

The design was set out in Polkadot's 2016 whitepaper by Gavin Wood, a co-founder of Ethereum, and is a central feature of Polkadot and its experimental sister network, Kusama. It offers an alternative both to building every application on one shared chain and to launching fully independent layer-1 blockchains that must attract their own validators. Similar ideas appear elsewhere under names such as appchains or subnets.

Think of a shopping centre. Each shop is free to choose its own layout, products and prices, but all of them share the centre's security guards, fire safety and car park. A new shop does not need to hire its own guards; it rents a unit and plugs into what is already there.

Parachains are the shops, and Polkadot's relay chain is the centre. Each parachain can be designed for one job, such as finance, gaming or identity, while relying on the relay chain's validators to keep it honest. Corridors between the shops let customers and goods move from one to another, which in blockchain terms means tokens and messages travelling between parachains.

• Collators. Each parachain has nodes called collators that gather its transactions and produce candidate blocks, along with a proof that the blocks follow the parachain's rules.
• Validation. Groups of relay chain validators are assigned to check those candidates. Once approved, the parachain block's summary is included in the relay chain and finalised.
• Shared security. Because the same validator set secures every parachain, attacking one parachain means attacking the whole network's stake, which is far more expensive than attacking a small standalone chain.
• Messaging. Parachains communicate using Polkadot's cross-consensus messaging format, known as XCM, enabling token transfers and contract calls between chains without external bridges.
• Custom runtimes. Most parachains are built with Substrate, a framework that lets developers choose their own fee models, governance and virtual machines, including Ethereum-compatible ones.

The relay chain can validate only a limited number of parachains at once, so access is rationed. From late 2021, projects competed in parachain slot auctions, bidding DOT that was locked for the length of the lease, up to two years. Many projects raised the necessary tokens through a crowdloan, in which supporters lent DOT in exchange for the project's own tokens and received their DOT back at the end of the lease.

Polkadot later moved away from long leases towards a model called agile coretime, introduced in 2024, in which projects buy blockspace, or "cores", for shorter periods or on demand. This lowers the cost of entry for smaller projects and lets established parachains scale their capacity up or down, and it means that being a parachain no longer requires winning an auction.

Parachains have been built for decentralised finance, smart contract platforms compatible with the Ethereum Virtual Machine, privacy, identity, gaming and connections to other networks such as Bitcoin and Ethereum. Because each chain can tailor its rules, a parachain focused on payments can, for example, let users pay fees in stablecoins, while one focused on identity can restrict who validates certain data. The model is a form of sharding in which each shard is a distinct chain with its own logic, rather than a copy of the same chain split for capacity.

Critics argue that slot auctions locked large amounts of capital and favoured well-funded teams, and that many crowdloan projects later struggled to sustain usage, leaving their tokens far below launch prices. Building a parachain requires more engineering than deploying a smart contract on an existing chain, which limited the number of developers. Fragmenting activity across many chains can also split liquidity and users, a problem that cross-chain messaging only partly solves. Polkadot has faced competition from Ethereum's layer 2 networks and other modular designs that offer similar shared security in different ways. Supporters counter that the parachain model gives projects sovereignty over their own rules while avoiding the security risks of standalone chains and third-party bridges. Holders of DOT who back validators, known as nominators, indirectly secure every parachain at once.