What is merged mining?

The short version

A SHA-256 miner spends its energy hashing block headers until it finds one below a target. Some chains, called auxiliary chains, agree to accept that same work as their own proof-of-work, as long as the parent block commits to the auxiliary block. That rule is AuxPoW (auxiliary proof-of-work). The miner hashes once; the pool checks each result against every chain's target and submits a block to each chain whose target it meets.

Merged mining does not split your hashrate. Your odds on the parent chain (eCash or Bitcoin) are exactly the same as without it. The merged coins are extra.

How it works, step by step

1. The pool asks each auxiliary chain's node for a block template and computes a short commitment (a hash) for each.
2. Those commitments are combined into one merkle root and written into the parent chain's coinbase transaction, the first transaction in the block your miner works on.
3. Your miner hashes as usual. It does not know or care that other chains are involved.
4. Every share that comes back is checked against the parent target and each auxiliary target. Auxiliary targets are usually much easier than the parent's.
5. When a share meets an auxiliary target, the pool sends that chain a block containing the parent header, the coinbase and a merkle proof. The auxiliary chain verifies that the work commits to its block and accepts it.

Because the commitment lives in the coinbase, the parent block is a normal, valid block. Parent-chain nodes see a few extra bytes in the coinbase and nothing else.

What MergedHash merge-mines

Coin	Paid to	Notes
Namecoin (NMC)	The key behind your bc1q address	The first merge-mined coin (2011). Import the same key or seed into a Namecoin wallet to spend.
Fractal Bitcoin (FB)	The key behind your bc1q address	Accepts merged-mined blocks for part of its block schedule.
Hathor (HTR)	Your H address, given in the password	Uses its own address format, so add htr=H… to your password.

Each merged block pays miners minus the same 1% fee. The live state of every coin (active, syncing, starting) is on the home page and the status page.

Solo and shared merged blocks

On a solo port, a merged block found by your share pays your address. On a shared port, merged blocks are split among the miners in the recent-shares window, the same way as parent blocks. Either way the reward is written into that coin's coinbase, so the pool never holds it. Merged coins have their own block times and difficulties, so you will usually see merged blocks more often than parent blocks.

Merged mining vs eCash sidechains

These are different mechanisms that are easy to mix up. Merged mining (AuxPoW) is when another chain accepts eCash or Bitcoin work as its own proof-of-work. eCash sidechains use blind merged mining (BMM, BIP301) under Drivechains (BIP300): sidechain operators pay eCash miners a fee to include a sidechain block hash in the eCash block, and the miner does not need to run the sidechain. Those BMM fees are part of the eCash block reward, and on MergedHash they go to the miners. See eCash sidechains.

Does it cost anything?

Not in hashrate or power. The cost is on the pool side: it runs a full node for every merged chain and keeps each one in sync. For miners the only visible change is a few more bytes in the coinbase, and more coins arriving at addresses they already control.

Merged mining questions

More guides: Bitaxe, NerdQAxe and NerdOctaxe · Antminer, Whatsminer and Avalon · Rented hashrate · Merged mining · Solo vs PPLNS · FAQ