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“The current Ethereum set-up has a blockchain consisting of a single chain with consecutive blocks. This is secure, but very slow and not efficient. With the introduction of shard chains, this blockchain is split up, enabling transactions to be handled in parallel chains instead of consecutive ones. This speeds up the network, and can scale more easily.”
Shard chains are the key to future scalability as they allow parallel transaction throughput and there will be 64 in Phase 1 (more maybe added as hardware improves; but this is unclear at this moment in time). Each shard chain is like one copy of the current ETH1 chain operating in parallel. The shard chains use the Beacon Chain for coordination.