What is a Chain Reorganization?

The meaning of chain reorganization refers to a phenomenon that can occur in blockchain-based systems, such as Bitcoin, when two different blocks are mined at roughly the same time, resulting in the creation of two competing versions of the blockchain.

When a new block is mined, it is broadcast to the network and other nodes in the network will add it to their local copy of the blockchain. However, if two blocks are mined at roughly the same time, it is possible that different nodes in the network will receive and add different blocks to their local copy of the blockchain. This can result in a temporary split in the network, with different nodes following different versions of the blockchain.

When this happens, the network will continue to mine new blocks on each of the competing chains, until one of the chains becomes longer than the other. The longer chain is then considered to be the valid version of the blockchain, and all nodes in the network will switch to following that chain.

The process of switching to a longer chain is known as chain reorganization. During this process, any transactions that were included in the shorter chain but not the longer chain will be considered invalid and will be removed from the network. This means that any transactions that were conducted during the time when the network was split may be temporarily reversed or invalidated.

Chain reorganization is a natural and expected part of the operation of blockchain-based systems, and it is designed to help ensure the integrity and security of the network. By allowing competing blocks to be created and resolved in a decentralized way, without the need for a centralized authority to resolve conflicts, blockchain-based systems are able to maintain their trustworthiness and robustness, even in the face of unexpected events or malicious actors.

Simplified Example

Imagine you are playing a game of Jenga, where you and your friends are taking turns removing blocks from a tall tower, one block at a time. The goal is to keep the tower standing for as long as possible.

In blockchain, each block contains a set of transactions that are verified and added to the chain. Just like how the Jenga blocks are stacked on top of each other, the blocks in the blockchain are linked together in a chain, with each new block added to the top of the previous one.

Now, imagine that one of your friends accidentally removes a block from near the bottom of the Jenga tower. Suddenly, the tower becomes unstable and starts to wobble, and some of the blocks on top fall off, creating a new, smaller tower.

In blockchain, a chain reorganization happens when a longer chain is created, causing the previous shorter chain to become obsolete. This can happen when two or more miners create new blocks at roughly the same time, resulting in two competing versions of the blockchain. The version with more blocks added to it will become the new longer chain and the other version will be discarded.

History of the Term Chain Reorganization

Chain reorganization in the context of blockchain technology refers to the temporary splitting and subsequent merging of the blockchain due to the creation of an alternate version of the chain. This phenomenon was notably observed in the early years of Bitcoin, particularly in 2010 and 2013. In 2010, a chain reorganization occurred when a block containing a large number of transactions conflicted with another chain, leading to a temporary split. Similarly, in 2013, another chain reorganization transpired due to a software bug that caused a fork in the blockchain, resulting in the temporary existence of two competing chains. These instances highlighted the importance of consensus mechanisms and the need for network stability and security in blockchain systems.


Fork: A fork is a change in the protocol of a blockchain network, resulting in the creation of two separate chains. For example, in 2017, the Bitcoin blockchain network underwent a hard fork, resulting in the creation of two separate chains: Bitcoin and Bitcoin Cash.

Reorg: A reorg is a short-term change to the blockchain. It occurs when a miner or group of miners take control of more than 50% of the network's hashrate, allowing them to reverse transactions.

Rollback: A rollback is a change to the blockchain that involves removing some blocks and returning to an earlier state. This is done to reverse a hack or a mistake in the system.

  • Blockchain Trilemma: The meaning of blockchain trilemma refers to the idea that it is difficult to simultaneously achieve the three key properties of a blockchain system: security, scalability, and decentralization.

  • Block Height: The meaning of block height refers to the number of blocks in a blockchain that have been added to the chain since its inception.