> ## Documentation Index
> Fetch the complete documentation index at: https://docs.abs.xyz/llms.txt
> Use this file to discover all available pages before exploring further.

# Handling Nonces

> Learn the best practices for handling nonces when building smart contract accounts on Abstract.

As outlined in the [transaction flow](/how-abstract-works/native-account-abstraction/transaction-flow), a call to `validateNonceUsage`
is made to the [NonceHolder](/how-abstract-works/system-contracts/list-of-system-contracts#nonceholder) system contract
before each transaction starts, in order to check whether the provided nonce
of a transaction has already been used or not.

The bootloader enforces that the nonce:

1. Has not already been used before transaction validation begins.
2. The nonce *is* used (typically incremented) during transaction validation.

## Considering nonces in your smart contract account

As mentioned above, you must "use" the nonce in the validation step.
To mark a nonce as used there are two options:

1. Increment the `minNonce`: All nonces less than `minNonce` will become used.
2. Set a non-zero value under the nonce via `setValueUnderNonce`.

A convenience method, `incrementMinNonceIfEquals` is exposed
from the `NonceHolder` system contract. For example, inside of your [smart contract wallet](/how-abstract-works/native-account-abstraction/smart-contract-wallets),
you can use it to increment the `minNonce` of your account.

In order to use the [NonceHolder](/how-abstract-works/system-contracts/list-of-system-contracts#nonceholder) system contract,
the `isSystem` flag must be set to `true` in the transaction, which can be done by using the `SystemContractsCaller` library shown below.
[Learn more about using system contracts](/how-abstract-works/system-contracts/using-system-contracts#the-issystem-flag).

```solidity theme={null}
// Required imports
import "@matterlabs/zksync-contracts/l2/system-contracts/interfaces/IAccount.sol";
import {SystemContractsCaller} from "@matterlabs/zksync-contracts/l2/system-contracts/libraries/SystemContractsCaller.sol";
import {NONCE_HOLDER_SYSTEM_CONTRACT, INonceHolder} from "@matterlabs/zksync-contracts/l2/system-contracts/Constants.sol";
import {TransactionHelper} from "@matterlabs/zksync-contracts/l2/system-contracts/libraries/TransactionHelper.sol";

function validateTransaction(
    bytes32,
    bytes32,
    Transaction calldata _transaction
) external payable onlyBootloader returns (bytes4 magic) {

    // Increment nonce during validation
    SystemContractsCaller.systemCallWithPropagatedRevert(
        uint32(gasleft()),
        address(NONCE_HOLDER_SYSTEM_CONTRACT),
        0,
        abi.encodeCall(
            INonceHolder.incrementMinNonceIfEquals,
            (_transaction.nonce)
        )
    );

    // ... rest of validation logic here

}
```