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A block is the fundamental container in a blockchain. It bundles together a set of validated transactions along with metadata that ties it to the rest of the chain.

Each block has two main parts:

• Header — metadata (timestamp, hashes, nonce)
• Body — the list of transactions

The header is small but powerful. A typical Ethereum-style header includes:

• previousHash — hash of the prior block
• timestamp — when the block was created
• merkleRoot — fingerprint of all transactions
• nonce — value used in consensus

The header is what gets hashed to identify the block.

Every block stores the hash of the previous block. This single field is what turns a pile of blocks into a chain.

Because each block points backward, the blocks form an ordered, append-only sequence reaching all the way back to the very first block.

The genesis block is the first block in any blockchain. It is special because it has no parent — its previousHash is typically all zeros.

Every other block can be traced back to genesis, which acts as the shared root of trust for the entire network.

The backward link makes tampering obvious. If someone alters a transaction in an old block, that block's hash changes.

But the next block stored the old hash in its previousHash field, so the link breaks. The mismatch is instantly detectable.

Blockchains are append-only data structures. You can add new blocks to the tip, but you cannot insert or delete blocks in the middle without breaking every link that follows.

This property is the foundation of blockchain immutability.

Block height is the number of blocks between a given block and the genesis block. Genesis is height 0, the next block is height 1, and so on.

Height gives every block a clear position and lets the network agree on which chain is longest.

The block body holds an ordered list of transactions. Order matters: it determines the final state after the block is applied.

Once a block is finalized, the transactions it contains — and their order — are fixed forever.

Blocks cannot grow without limit. Each chain sets a cap — Bitcoin uses a byte limit, while Ethereum caps the total gas a block may consume.

These limits keep blocks small enough to propagate quickly across the network.

A block's identity is its hash — the output of running its header through a cryptographic hash function.

Change any field in the header and the hash changes completely. That is why hashes act as tamper-evident fingerprints for blocks.

A blockchain is simply a linked list of blocks where each link is a cryptographic hash of the prior block.

This combination of ordering, hashing, and append-only writes is what makes the chain trustworthy without any central authority.

Test your understanding of how blocks connect.

You learned that:

• A block has a header (metadata) and a body (transactions)
• Blocks link by storing the previousHash
• The genesis block is the root with no parent
• Chains are append-only, making them immutable
• A block's identity is its hash

Next, we explore how Merkle Trees summarize all transactions efficiently.