> ## Documentation Index
> Fetch the complete documentation index at: https://docs.polygon.technology/llms.txt
> Use this file to discover all available pages before exploring further.

# Polygon Chain overview

> Polygon Chain is an EVM-compatible blockchain anchored to Ethereum. Sub-5 second finality, 3,800 TPS, and $0.002 average transaction cost. Architecture, finality, and core contracts explained.

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.

## 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](https://docs.cometbft.com/) 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.

## 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.

<Note>
  Checkpoint verification is required for bridging assets from Polygon Chain to Ethereum. For transactions that stay on Polygon Chain, milestone finality applies instead.
</Note>

## 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.

## 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.