# Pool Initialization (/docs/protocols/uniswap-labs-hooks/erc4626-wrapper/concepts/pool-initialization)

Learn the conditions ERC4626WrapperHook enforces before a wrapper pool can exist, and how swap direction maps to wrap or unwrap.

`_beforeInitialize` enforces two conditions before the pool can exist:

1. `currency0` and `currency1` are the wrapper and underlying currencies, in the order fixed by `wrapZeroForOne` (`InvalidPoolToken`).
2. `fee == 0` (`InvalidPoolFee`).

`wrapZeroForOne` is an immutable set in the constructor to `underlyingCurrency < wrapperCurrency` (by address). It records which token is `currency0`:

* `wrapZeroForOne == true`: `currency0` is the underlying, `currency1` is the wrapper. A `zeroForOne` swap wraps.
* `wrapZeroForOne == false`: `currency0` is the wrapper, `currency1` is the underlying. A `zeroForOne` swap unwraps.

This maps a swap direction to wrap or unwrap deterministically, regardless of how the two token addresses sort.