Finality and Fork Handling

What 'Final' Actually Means

When you send a transaction, it gets included in a block, and that block becomes part of the chain. But early on, it's possible that a different, conflicting block could replace it (a 'reorg') — your transaction would be undone. As more blocks pile on top, this gets exponentially less likely. 'Finality' is the point at which the protocol guarantees your transaction won't be reverted. Different chains define and achieve finality differently.

Probabilistic vs Absolute Finality

Bitcoin uses probabilistic finality. Every block makes it more expensive for an attacker to revert your transaction; the convention is to wait ~6 blocks (about 1 hour) for high-value transactions. There's no point where Bitcoin guarantees finality; it's just probability getting closer and closer to certainty. Ethereum and most modern chains add absolute finality — once enough validators have voted to finalise a block, the protocol guarantees it can never be reverted (unless validators are willing to lose huge amounts of staked ETH).

Why It Matters

If you're a user buying coffee, probabilistic finality is fine — coffee shops can wait. If you're an exchange crediting a deposit, you need confidence: a reorg that undoes your customer's deposit while they've already withdrawn the equivalent elsewhere is a real loss. Different chains' finality models drive what activities are safe to do quickly vs. need to wait.

• Probabilistic finality: each block makes revert harder; Bitcoin (PoW), some others
• Absolute finality: a specific point where protocol guarantees no revert; Ethereum (post-Merge), Cosmos, Solana, most modern chains
• Both work; the trade-offs are about speed and economic security
• Reorgs are normal at the 1-2 block depth; rare past 6 blocks in Bitcoin; nearly impossible past 64 epochs in Ethereum