In plain English
What Layer 1 means
Layer 1 is the primary blockchain itself, such as Bitcoin or Ethereum. Other systems, including layer-2 networks, can build on top of it, but the layer-1 chain is the base ledger that handles core validation and final settlement rules.
Why it matters
Knowing what is layer 1 helps distinguish the base network from scaling layers, side systems, and token bridges. Fees, speed, and security assumptions often differ between the base chain and what is built on top of it.
Example
Ethereum mainnet is layer 1. A rollup that posts data back to Ethereum is a separate scaling system built on top of that layer-1 base.
Quick answers
Common questions
Is layer 1 the same as the mainnet?+
Usually yes, when people are talking about the live base blockchain rather than a test network.
Does layer 1 always mean Bitcoin or Ethereum?+
No. Any base blockchain in a multi-layer design can be called layer 1.
Sources