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 impact?
One of simplest ans is
draw the black line according to your requirement using nine patch tools.
You can find more information about how nine patch works here
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.
Here http://developer.android.com/guide/topics/graphics/2d-graphics.html#nine-patch you can find a very good explanation, with images to show you that the left and top part of the black border indicate a stretchable section, while the right and bottom lines define the drawable area, where you can put your content.
来源：https://stackoverflow.com/questions/4517693/how-does-a-9patch-png-work-in-android-apps