What is Nonce?

In Ethereum, every transaction has a nonce. The nonce is the number of transactions sent from a given address.

Each time you send a transaction, the nonce increases by 1. There are rules about what transactions are valid transactions and the nonce is used to enforce some of these rules. Specifically:

• Transactions must be in Order: You cannot have a transaction with a nonce of 1 mined before one with a nonce of 0.

• No Skipping! You cannot have a transaction with a nonce of 2 mined if you have not already sent transactions with a nonce of 1 and 0.

Why?

This field prevents double-spends as the nonce is the order the transactions go in. In order for a double-spend to be "successful", you typically...

• Send a transaction to one party

• Wait for it to register

• Collect something in return from this first transaction

• Quickly send another transaction with a high gas price

• Have this second transaction mined first, therefore invalidating the first transaction.

This is why exchanges wait for you to have a certain number of confirmations before allowing you to trade freshly-deposited funds.

In Ethereum, the above is not possible

In Ethereum, this method of "double-spending" is not possible because each transaction has a nonce included with it. Even if you attempt to do the above, it will not work as the second transaction (nonce of 3) cannot be mined before the first transaction (nonce of 2).