Overview
The blockchain is Bitcoin's core data structure: an ordered, ever-growing sequence of blocks, each cryptographically linked to its predecessor. This chaining mechanism ensures that any attempt to alter historical data would require redoing the proof of work for the modified block and every subsequent block, making tampering computationally infeasible.
Structure
┌──────────┐ ┌──────────┐ ┌──────────┐ ┌──────────┐
│ Block 0 │←───│ Block 1 │←───│ Block 2 │←───│ Block 3 │
│ (Genesis)│ │ │ │ │ │ (Tip) │
│ │ │ prev: │ │ prev: │ │ prev: │
│ hash: a1 │ │ a1 │ │ b2 │ │ c3 │
│ │ │ hash: b2 │ │ hash: c3 │ │ hash: d4 │
└──────────┘ └──────────┘ └──────────┘ └──────────┘
Each block stores the hash of the previous block's header,
creating an unbreakable chain back to the genesis block.
Key Properties
- Append-only: New blocks are only added to the tip; existing blocks are never modified
- Tamper-evident: Changing any data in a historical block would change its hash, breaking the chain from that point forward
- Distributed: Every full node maintains an independent copy of the entire blockchain
- Transparent: All transactions are publicly visible and verifiable by anyone
The Timechain
Some Bitcoiners prefer the term "timechain" (a word Satoshi used in early communications) to distinguish Bitcoin's specific implementation from the broader, often overhyped concept of "blockchain technology." The timechain emphasizes the temporal ordering aspect: Bitcoin's blockchain is fundamentally a decentralized timestamping system that establishes the chronological order of transactions.
Size and Growth
The Bitcoin blockchain grows by approximately 50-80 GB per year, depending on how full blocks are. As of 2025, the full blockchain exceeds 600 GB. This size is manageable for consumer hardware, which is important for maintaining decentralization by keeping full node operation accessible.