What flour is used to make white sauce?

Here's how it's made:

1. Butter is melted in a saucepan.

2. Flour is whisked into the melted butter to create a roux.

3. Milk is slowly whisked into the roux while constantly stirring to prevent lumps.

4. The sauce is simmered until thickened.

So, while flour is a key ingredient in the roux, it isn't the flour itself that makes the white sauce. The roux is simply a thickening agent, and the main ingredient in white sauce is milk.