In the washer method, each "washer" is a rectangle rotated about the axis. So the inner radius is the edge of the rectangle close to the axis of rotation, and the outer radius is the edge of the rectangle away from the axis of rotation.

It's really similar to finding the area between two curves - which curve gets the minus sign?

I feel like I haven't explained it very well - does anyone else want to give it a try?

- Hollywood