TECHNICAL_DOC // BLOCKCHAIN / LONGEST-CHAIN
LONGEST
CHAIN
CHAIN
Bitcoin nodes follow the chain with the most accumulated proof of work —
commonly called the "longest chain" rule, though more precisely it is the
heaviest chain (most total difficulty). This rule is Bitcoin's decentralized
consensus mechanism: honest miners extend the heaviest chain, making it progressively
harder for any attacker to rewrite history.
CHAIN_SELECTION_RULE
HEAVIEST CHAIN — NOT JUST BLOCK COUNT
Chain weight = sum of difficulty for every block
Example: two competing chains
Chain A: 900 blocks, avg difficulty = 1.0T → weight = 900T
Chain B: 850 blocks, avg difficulty = 1.1T → weight = 935T
Winner: Chain B (higher total difficulty, fewer blocks)
Nodes always switch to the chain with highest total work
regardless of block count.
In practice: difficulty retargets every 2016 blocks, so
longer chains almost always have more total work unless
there's a dramatic difficulty drop mid-fork.
Bitcoin Core tracks chainwork as uint256 cumulative
difficulty target in the block index database.
WHITEPAPER
Satoshi's whitepaper describes the chain selection rule as the foundation of Bitcoin's consensus — honest nodes always extend the longest (heaviest) chain, forming a majority that outpaces any attacker.
From the Bitcoin Whitepaper (Section 5):
"The longest chain not only serves as proof of the sequence
of events witnessed, but proof that it came from the largest
pool of CPU power."
Key insight: if honest nodes control majority hashrate:
→ honest chain grows faster than any attack chain
→ attack chain eventually falls behind
→ nodes reject shorter (lower-work) competing chain
The rule is self-enforcing:
- Miners waste resources if they mine on a shorter chain
- Rational miners always build on the heaviest chain
- Game theory favors honesty when attacker < 50%
The "longest chain" terminology is technically imprecise. Bitcoin Core uses GetBlockProof() to compute cumulative chainwork as the true selection criterion.
TERMINOLOGY_INDEX
Longest Chain
Common name for the chain selection rule. More precisely: the chain with the most cumulative proof of work.
Chainwork
The sum of all block difficulties in a chain from genesis to tip. Stored as uint256 in Bitcoin Core.
Nakamoto Consensus
Bitcoin's decentralized agreement mechanism: nodes follow the heaviest chain, enforced by economic incentives.
Heaviest Chain
The technically correct term for the chain Bitcoin nodes prefer — total work, not block count.