Merge

The whitepaper flagged this leak in 2008, warning that multi-input transactions "necessarily reveal that their inputs were owned by the same owner". Researchers later turned that sentence into the common-input-ownership heuristic, which has been the backbone of wallet clustering since Meiklejohn's 2013 study of the bitcoin economy. A merge is not an attack on you; it is your own wallet filing a statement.

How it works

Each input in a bitcoin transaction is unlocked separately, and the proof of that is written into the transaction itself.

To spend one coin you produce a signature from the key that controls it. To spend two coins in the same transaction you produce two signatures, one per input, which means one signing party had access to both keys at the same moment. There is no way to hide it: the signatures are part of the data every node validates and every explorer displays. The link is not a probability, it is a structural fact, which is why analysts trust this heuristic more than any other rule they apply.

The consequences run in both directions. Backwards, the merged coins drag their separate histories into one cluster, so a coin traceable to a verified exchange account pulls the other coin into the same identified group. Forwards, whoever you paid can add up the value of every input you spent and see how much you were holding in that part of your wallet, not just the amount of their invoice.

Only two designs deliberately break the rule. A coinjoin puts unrelated people's inputs into one transaction so the heuristic produces a wrong cluster, and a payjoin does the same on a small scale during a normal payment. Everything else your wallet does obeys it.

Where you see it

Merges usually happen automatically, at the exact moment you are not thinking about privacy.

Coin selection is the main culprit. When no single coin covers the amount you are sending, the wallet grabs a second one without asking, and the merge is published. "Send max" sweeps the whole wallet into one transaction and merges everything you own. Sweeping an old paper wallet or a gift card into your main wallet merges that coin into your cluster too, and any consolidation is a merge you performed on purpose.

Sometimes merging is the honest choice. Bitcoin Core added the -avoidpartialspends option in version 0.17.0, which always spends every coin sitting on the same address together, even when that costs more in fees. The logic is that coins paid to one reused address are already linked in public, so keeping them apart buys nothing while spending them separately at different times leaks extra information.

The way to see merges before they happen is coin control: turn it on, look at which coins the wallet has ticked, and untick the ones you would rather leave alone.

Merge vs the Ethereum Merge

Searching for "the merge" mostly returns a different network's history. The Merge was Ethereum's switch from proof of work to proof of stake on September 15, 2022, a consensus change that retired Ethereum mining overnight. Bitcoin has had no comparable event and still secures its chain with proof of work. In a bitcoin context, merge always means combining coins inside one transaction, and the only thing at stake is privacy.