Coincident Blocks
How merge-mined blocks are used to transfer state across blockchains in Quai Network.
The Natural Bridges Between Chains
Imagine rolling dice and occasionally getting lucky enough to win multiple games at once. That’s essentially what coincident blocks are - mining solutions that satisfy multiple blockchain requirements simultaneously.
What Makes Them Special:
No extra work required: They occur naturally through normal mining
Mathematically guaranteed: Not reliant on validators or committees
Perfect synchronization: Chains stay connected through pure probability
How They Enable Cross-Chain Transactions: When a coincident block occurs, it creates an unbreakable mathematical link between chains. This link serves as a trustless bridge, allowing assets and data to move between chains without any intermediary.
The Mathematics of Luck
Simple Analogy:
Zone difficulty: Need to roll a 6
Region difficulty: Need to roll two 6s
Prime difficulty: Need to roll three 6s
Sometimes you roll three 6s when you only needed one - that’s a coincident block!
Probability Table:
Slightly lucky (1 extra zero)
50%
Pretty lucky (2 extra zeros)
25%
Very lucky (3 extra zeros)
12.5%
Extremely lucky (4 extra zeros)
6.25%
Why This Matters: Unlike other multi-chain systems that rely on committees, validators, or complex protocols, Quai’s cross-chain bridges are created by pure mathematics. No trust required - just probability.
How Coincident Blocks Work
Key Principles:
Hierarchy matters: Prime blocks are always coincident (they satisfy all chains)
Independence preserved: Zones can produce blocks without waiting
Atomic validity: A block must be valid everywhere or nowhere
Perfect synchronization: All chains append coincident blocks simultaneously
The Chain Hierarchy:
Prime blocks: Always coincident (like a master key opening all locks)
Region blocks: Coincident with their zones
Zone blocks: Usually independent, occasionally coincident
Why Different Block Times?
Prime: ~20 seconds (slow but ultra-secure)
Region: ~10 seconds (balanced)
Zone: ~5 seconds (fast for users)
This design ensures zones can operate quickly while still maintaining periodic synchronization through coincident blocks.
Running a Node
Minimum Requirement - A “Slice”: The smallest node a miner can run is a slice node. To participate trustlessly, you need:
The prime chain (global coordination)
One region chain (regional coordination)
One zone chain (where transactions happen)
This “slice” gives you everything needed to verify transactions without trusting anyone else.
The Beauty of the System: Coincident blocks create a trustless, automatic, and mathematically guaranteed way for multiple blockchains to stay synchronized and share security - all through the natural randomness of mining.
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