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Polygon Chain is an EVM-compatible blockchain anchored to Ethereum. It increases Ethereum’s effective throughput and reduces transaction costs by executing transactions off the Ethereum mainnet and periodically anchoring state to it through a checkpoint mechanism.

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Dual-layer architecture

Polygon Chain is composed of two layers that operate together:
  • Heimdall (consensus layer): a set of proof-of-stake nodes that monitor staking contracts on Ethereum, validate Bor block data, and submit periodic checkpoints to the Ethereum mainnet. Heimdall-v2 is built on CometBFT and Cosmos SDK.
  • Bor (execution layer): a set of block-producing nodes that execute EVM transactions. Bor is based on Go Ethereum (Geth), with Erigon also supported. Each span has a single block producer, selected by Heimdall from a small pool of eligible validators.
The two layers are designed around separation of concerns: Bor handles fast transaction execution, while Heimdall handles consensus, finality, and Ethereum anchoring.

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How transactions reach finality

With Heimdall v2, deterministic finality is achieved in 2 to 5 seconds through a milestone mechanism. Validators use vote extensions on the Heimdall consensus layer to agree on the longest common sequence of Bor block hashes since the last finalized milestone. When 2/3 or more of the validating stake agrees, that sequence is finalized as a new milestone. Milestones and checkpoints serve different purposes:
  • Milestones finalize blocks on the Polygon chain itself, within seconds, using the Heimdall consensus layer.
  • Checkpoints are Merkle roots of Bor block data submitted to Ethereum at intervals set by a governance parameter. They provide proof of state on Ethereum and are required for withdrawals to the mainnet.
For L2-to-L2 transfers, finality is provided by milestones and does not require waiting for a checkpoint.
Checkpoint verification is required for bridging assets from Polygon Chain to Ethereum. For transactions that stay on Polygon Chain, milestone finality applies instead.

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Core contracts on Ethereum

A set of contracts on the Ethereum mainnet connect Polygon Chain to Ethereum. These contracts handle:
  • Staking: validators lock POL tokens here to participate in consensus.
  • Checkpoint storage: Heimdall submits Merkle roots of Bor block data to these contracts periodically.
  • Asset bridging: users who want to withdraw tokens to Ethereum interact with an exit queue managed by these contracts.
The exit queue enforces a secure, orderly process for asset withdrawals. A withdrawal can only be finalized on Ethereum after the relevant checkpoint has been submitted and verified.

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Validator nodes

Validators run both a Heimdall node and a Bor node. Their responsibilities span two areas:
  • On Heimdall: monitoring staking events on Ethereum, validating Bor blocks, and proposing or co-signing checkpoints.
  • On Bor: producing and validating blocks.
Validators stake POL tokens on Ethereum. A minimum of 10,000 POL is required (raised by governance proposal PIP-4). The network supports up to 105 active validators at any time.