Where six confirmations came from
The original Bitcoin paper works out the chance that an attacker holding some share of mining power catches up and rewrites the chain from a given depth. The chance collapses quickly as blocks pile on top. At around six blocks it becomes small enough, under the assumptions in that section, to stop thinking about.
That is the entire origin. A worked example under stated assumptions, in one document, about one chain. It was never adopted as a rule and no software enforces it. Exchanges picked it up, wallets printed it in their interfaces, and it turned into folklore that people now quote back at each other.
The page marks the number judged for exactly that reason. The arithmetic behind it is real. The choice of where to stop was somebody deciding a probability was small enough, and different people making that call honestly would land in different places.
What knowing the six is good for
It explains why waiting feels arbitrary. If you have ever wondered why a service made you wait for a number it never justified, this is usually the answer. The number was inherited rather than worked out for the case in front of it.
It gives you a way to read a policy. A service demanding many confirmations on a tiny payment is being cautious with no benefit to anyone. A service crediting a very large payment after one is taking a risk you may not want to be part of, and since a settled transfer does not come back, that risk lands somewhere. The right depth scales with what is at stake, and six fits a lot of ordinary cases by accident rather than design.
It also separates two ideas people run together. Confirmations are about how hard a record would be to rewrite. Locks are about whether coin can be spent yet. The ten block page is about the second and has nothing to do with this one.
Treating six as a standard
The first mistake is expecting it. Markets and exchanges pick their own threshold and it is frequently one, two or three. If a deposit credits after three confirmations, nothing has been cut short and nobody is being careless with you.
The second is carrying the number between chains. Six blocks on a chain with ten minute blocks is an hour of accumulated work. Six blocks on a chain with two minute blocks is twelve minutes, a fifth of the time for the same word. The unit reads the same and means something different.
The third is confusing it with the Monero spend lock. That lock is a consensus rule applying to every received output on the chain and exists for reasons unrelated to the probability table above. One is a habit an operator changes by editing a setting. The other is written into the rules everybody runs.
| Threshold | Where you meet it | What is behind it |
|---|---|---|
| 1 | Small deposits | Almost always enough in practice |
| 3 | Common crediting threshold | A compromise costing a few minutes |
| 6 | Exchanges and larger amounts | The inherited convention from 2008 |
| 10 | Monero spendability | A consensus rule, not a policy |
What sets the number instead of six
The amount at stake comes first. Rewriting recent chain history costs real money to attempt, so the sensible depth is the one where the cost of the attempt exceeds what the attacker would win. For an ordinary order that depth is very shallow indeed.
Then the chain itself, since block time and total mining power both change what one block of depth is worth. Then the operator, who is balancing risk against complaints about waiting and will usually pick a round number and leave it untouched for years. None of those three inputs is the paper that started it.
Questions people ask
Does Monero use six confirmations?
No. Monero has a ten block spend lock in consensus, and services built on top pick their own crediting threshold. The ten block page covers the difference.
Is waiting longer always safer?
Marginally, with sharply falling returns. Past a handful of blocks the extra safety is smaller than the inconvenience for almost any ordinary amount.
Where can I read the original argument?
It sits in the closing section of the 2008 Bitcoin paper, which works through the probability of a chain rewrite at increasing depth.