Historically, before rollups existed, anything that helped scale Ethereum was loosely called “Layer 2.”
This included:
- sidechains (Polygon PoS)
- plasma chains
- state channels
But over time, the definition of a true Layer 2 has become much stricter.
✔️ Modern Definition of a True Layer 2
A true L2 must:
- Inherit security from the L1
- Have transaction data available on the L1
- Have trustless escape hatches (users can withdraw even if the L2 fails)
- Use the L1 as its final settlement layer
Rollups fit this definition.
Sidechains do NOT.
🔥 So why are sidechains still sometimes called L2?
Because:
- They interoperate with the L1
- They scale transaction throughput
- They bridge assets to/from the L1
- They offer a similar user experience (cheap, fast transactions)
But these points don’t make them true L2.
They are really parallel L1s connected by a bridge.
🧩 What is the actual link between a sidechain (e.g., Polygon PoS) and the L1 (Ethereum)?
Sidechains connect to an L1 through a bridge, NOT through security or state verification.
The link is:
🔗 1. Asset bridge
- Users lock tokens on Ethereum
- Corresponding tokens are minted on Polygon PoS
- The bridge tracks ownership
- Withdrawals rely on multisigs or validators, NOT Ethereum consensus
🔗 2. Occasional state checkpoints
Polygon PoS periodically sends a checkpoint hash to Ethereum.
BUT — important:
➡️ Ethereum does NOT verify Polygon PoS transactions
➡️ It only stores a hash
➡️ Validation happens on Polygon, not on Ethereum
This means Ethereum cannot:
- verify block validity
- detect fraud
- recover user funds
- enforce withdrawals
Because Polygon PoS runs its own consensus separate from Ethereum.
🔗 3. Validator staking
Polygon validators stake MATIC — not ETH — on Ethereum.
This is an economic link, not a security link.
In short:
👉 Polygon ≠ secured by Ethereum
👉 Polygon = uses Ethereum as a funding + checkpoint hub
🧠 Key Insight: Sidechains don’t inherit L1 security — they only connect to L1.
This is the crucial difference.
Rollups
✔ All data + proofs go to Ethereum
✔ Ethereum can reconstruct the L2 state
✔ Ethereum enforces fraud/validity
✔ Users can withdraw trustlessly
Sidechains
✘ Data stored on the sidechain
✘ Ethereum can’t verify anything
✘ Bridges depend on a multisig or validator quorum
✘ If the sidechain or bridge is hacked, funds are gone
(see: Ronin $600M hack, Harmony $100M hack)
This is why sidechains are not considered true L2s today.
🟦 Case Study: Polygon PoS vs Polygon zkEVM
| Feature | Polygon PoS (Sidechain) | Polygon zkEVM (True L2) |
|---|---|---|
| Security | Own PoS validators | Ethereum via ZK proofs |
| Data Availability | On Polygon | On Ethereum |
| Bridge trust | Trusted | Trustless |
| Withdrawal security | Dependent on validator honesty | Guaranteed by L1 |
| Settlement | On Polygon | On Ethereum |
| Category | Sidechain | ZK Rollup (L2) |
Polygon PoS is a sidechain.
Polygon zkEVM is an L2 rollup.
Polygon the ecosystem includes both — this is the confusing part.
🧠 Why people mistakenly call Polygon PoS an L2
Because Polygon markets itself as:
“Ethereum Scaling Solutions”
…but not all of its products are true L2.
Polygon PoS scales Ethereum economically, but not securely.
That’s why:
- Developers treat it like an L2 (cheap, fast)
- Researchers classify it as a sidechain
- Polygon themselves now label it a “commit chain,” not an L2
📌 TakeAway
Sidechains (like Polygon PoS) are sometimes called L2s because they help scale Ethereum and are bridged to it.
But technically, they are not L2 because they don’t inherit Ethereum’s security; they rely on their own consensus.
The link between a sidechain and an L1 is only a bridge and optional checkpointing — NOT shared security.
