Stale Block

A stale block is a valid block that was successfully produced but is not part of the chain the network finally settles on. It usually arises when two miners find a block at almost the same height at nearly the same time: both blocks are valid, but only one can become the parent of the next block, and the other is left behind. Its producer loses the block reward, and its transactions return to the queue unless they also appear in the winning block.

Stale blocks are a normal by-product of networks where blocks spread from computer to computer with some delay, especially under proof of work. They are widely called "orphan blocks", but in Bitcoin's own technical vocabulary that term means something different, and the mix-up is one of the most common in the field. On proof-of-work Ethereum, stale blocks had a third name, uncles or ommers, and earned a partial reward.

Imagine two runners in a relay who both grab the baton handed over at the same moment and set off along slightly different paths. Officials let both run, but as soon as the next runner takes the baton from one of them, that path becomes the official route. The other runner finished the leg properly, yet the effort no longer counts.

A stale block is that second runner. Nothing was wrong with it; it just lost a race against time. The network does not punish it, but it does not reward it either, and any entries written only on that block need to be written again on the official route.

A new block takes time, typically a few seconds, to reach every node on the network. If a second miner finds a competing block during that window, some nodes see one block first and some the other. The network is temporarily split in a small, natural fork.

Each node follows the valid chain with the most cumulative work and, between equal branches, usually keeps the one it saw first. The tie breaks when the next block is found on top of one branch: that branch now has more work, every node switches to it, and the other block becomes stale. This switch is called a chain reorganisation, or reorg. Deeper reorgs are possible but become rarer with each block, which is why recipients wait for several confirmations and why Bitcoin's finality is described as probabilistic.

A common rough approximation of the chance that a block goes stale is:

• $P_{\text{stale}}$ is the probability that a competing block is found before the new block has spread.
• $t$ is the time the block takes to propagate across the network, in seconds.
• $T$ is the average time between blocks, in seconds.

Worked example: with Bitcoin's $T = 600$ seconds and a propagation time of $t = 2$ seconds, $t / T \approx 0.0033$, so $P_{\text{stale}} \approx 1 - e^{-0.0033} \approx 0.33\%$, or about one block in 300. Shorter block times or slower propagation raise the rate, which is one reason faster chains need different designs.

The Bitcoin developer documentation draws a clear line between the two terms:

• A stale block is a block that was successfully mined but is not on the current best chain.
• An orphan block is a block whose parent the local node has not yet received, so it cannot be fully checked yet. The block is "orphaned" only in the sense that its parent is missing; once the parent arrives, it may join the main chain.

Early Bitcoin software kept an "orphan pool" of such parentless blocks, and many users, websites and even some block explorers began calling every losing block an orphan. Since Bitcoin Core moved to downloading block headers before full blocks in 2015, true orphan blocks have become rare, while stale blocks still occur. In everyday usage "orphan block" almost always means a stale block, and readers should check which sense a source intends.

Ethereum's original design targeted a block roughly every 13 to 15 seconds, so stale blocks were far more frequent than on Bitcoin. To avoid penalising small or distant miners, the protocol let later blocks reference recent stale blocks as "uncles" (later renamed ommers). The uncle's miner received a reduced reward and the including miner a small bonus, and the uncles' work counted toward choosing the heaviest chain.

With the move to proof of stake in September 2022, ommer rewards were abolished. Under the current design one validator proposes per 12-second slot, so competing blocks are less common, though a late block can still be skipped by the next proposer, leaving it out of the canonical chain; such blocks are often called "orphaned" or "reorged" blocks.

For the miner, a stale block is lost income, which favours large, well-connected mining pools that hear about new blocks first. Relay improvements such as compact block relay have cut Bitcoin's stale rate considerably. For users, a payment that sat only in the stale block falls back into the mempool, and a sender could exploit the confusion by broadcasting a conflicting payment, the risk behind double spending against merchants who accept a zero-confirmation transaction.

Researchers have also shown that miners can create stale blocks on purpose. In the "selfish mining" strategy described by Ittay Eyal and Emin Gün Sirer in 2013, a miner withholds blocks it has found and releases them strategically, turning honest miners' blocks stale and earning more than its share of hashrate would justify. Monitoring stale rates is one way analysts look for such behaviour and for signs of a developing 51% attack.

• Bitcoin Developer Glossary
• Bitcoin Developer Guide: Block Chain