how does a 9patch png work in android apps

Question

In a 9patch png, there are black edges in its four sides, but there is usually difference between the left to right, and the up to down side. Why? How does this difference i

Answer 1

The left and top are for scaling, and the right and bottom are for content. So the black lines on left and top will scale to fit, and the content for a button will be "inside" the black lines on the right and bottom.