# Block¶

The central element of every cryptocurrency is a public ledger called the blockchain, which links blocks together.

Since the blocks in the chain are ordered, the complete transaction history is held on the blockchain. Subsequent blocks have increasing heights that differ by one. Each block can contain up to 1,000 transactions (public network), being this value configurable per network.

Symbol blocks complete every 15 seconds, making transactions confirm quickly enough for everyday use.

## Creation¶

Symbol calls the first block in the chain the nemesis block. The first block is defined before launching a new network and sets the distribution of the network currency mosaics.

The process of creating the subsequent new blocks is called harvesting.

Blocks are created by accounts, which are chosen by the consensus algorithm based on their importance score. The consensus algorithm determines a new account to harvest the subsequent block after each block creation.

The harvesting account receives the fees for the transactions added in the block and the mosaics created by inflation. This gives the harvester an incentive to add as many transactions to the block as possible.

## Rollbacks¶

Blockchains are designed in a way that, in the presence of a network failure or partition, the recent blocks might need to be rolled back.

The rewrite limit is the maximum number of blocks that can be rolled back. Hence, forks can only be resolved up to a certain depth too.

Symbol’s public network has a rewrite limit of 398 blocks, but this limit is configurable per network. The transactions linked to a block are permanently recorded on the blockchain once the number of confirmation blocks (subsequent blocks) surpasses the maximum number of rollback blocks.

## Guides¶

Continue: Transaction.