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# Algorand Specifications

The [_previous hash_](https://specs.algorand.co/ledger/ledger-block#previous-hash) is a cryptographic hash of
the previous block header in the sequence of blocks.

The sequence of previous hashes in each block header forms an authenticated, linked-list
of the reversed sequence.

Let Br represent the block header in round r, and let Hash be some cryptographic hash function.

Then the previous hash in the block for round r+1 is
Prevr+1=Hash(Br).

> In the reference implementation, Hash is the [SHA512/256 hash function](https://specs.algorand.co/crypto/crypto-sha512-256).