What Is an Orphan Block?

In blockchain terms, orphan blocks are blocks mined simultaneously as another block but not accepted by the blockchain. Most of the time, this is because there are not enough blocks generated from that block for the network to recognize it as the longest fork. The Bitcoin blockchain discards orphan blocks; however, other blockchains may use them for different purposes.

Technically, orphan blocks are called stale blocks, but because most people refer to them as orphans or uncles, the name orphan (and uncle) block has stuck.

Understanding Orphan Blocks

A blockchain consists of a series of blocks, which act as data storage units to store details of the various transactions occurring on the blockchain network. During the standard mining process, miners attempt to generate new blocks by solving the hash—the hexadecimal number that stores the block's information.

The first miner who successfully opens a new block is entitled to the block reward and writes the first transaction on the new block. The newly opened block stores information about the previous blocks and new transactions and is mined to open another block.

How Does a Block Become Orphaned?

The series of blocks that create a blockchain are related in that they receive information from the blocks that preceded them. When a block is closed, its data is encoded and passed on to the next block. These two blocks are a parent and child block. If two blocks are opened from the same parent block simultaneously, there are two child blocks. Only one of them can be integrated into the chain.

The network nodes, which validate blocks, decide which block to use by allowing a small fork between the two child blocks. Then, the nodes determine what block they want to accept by reaching a validation consensus.

Each block will have subsequent blocks created, initiating a race to verify the most blocks. The fork with more verified blocks—through proof of work (PoW)—gets accepted into the blockchain. Any verified blocks within the shorter chain are discarded.

The discarded block is called an orphan block (in technical documents, it's called a stale block). Any blocks generated from the orphaned block go back to the memory pool to be validated and added to the new chain.

How Are Orphan Blocks Different From Stale Blocks?

As mentioned previously, many people call blocks rejected by the network orphan blocks. In a familial relationship, this is not the correct term. Because blocks are referred to by ancestral relationships for easy reference, an orphan block would technically be a block with unknown parent blocks.

A block without a parent block is one with an incomplete block hash. The block hash is an encrypted number and a snapshot of the complete blockchain at the moment the block was created. Parent block information would be included in this hash, so an orphan block would be a strange occurrence in a network that relies on validation and verification of all preceding blocks.

An actual orphan block without any parental block information is likely a block that has been tampered with. However, for simplicity, an unaccepted block is called an orphan block.

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