sidechain

A sidechain is a fully independent blockchain that runs alongside a main chain and is connected to it by a two-way bridge. It has its own blocks, its own consensus, and its own validators, and it inherits none of the main chain's security. Think of it as a neighbouring country with its own government and its own police force: you can move money across the border through a currency exchange, but once you are inside, you live under that country's laws, not your home country's. Polygon's original Proof-of-Stake chain and Gnosis Chain are well-known Ethereum sidechains.

The peg works by lock-and-mint. To move an asset onto the sidechain you lock it in a bridge contract on the main chain, and an equal amount is minted on the sidechain; to come back, you burn the sidechain tokens and unlock the originals. This is the crucial distinction from a rollup: a sidechain does not post its transaction data or proofs to the main chain, so the main chain cannot verify or enforce the sidechain's state. The bridge is secured only by the sidechain's own validator set (or a multisig), which is exactly why sidechain bridges have been among the largest hacks in the space.

The trade-off is straightforward. Because a sidechain runs its own consensus with fewer or differently-incentivized validators, it can offer high throughput and very low fees — but its safety rests entirely on those validators behaving, not on Ethereum. If the sidechain's consensus is corrupted or its bridge multisig is compromised, your bridged funds can be stolen and Ethereum will neither know nor help. Rollups were designed precisely to fix this gap by inheriting the base layer's security; sidechains remain useful where independent sovereignty and raw performance matter more than trust-minimized security.