You know that bottom checkbox that you never use when you're creating a symbol? The one that enables guides for 9-slice scaling?Well if you've never used it before and you have the need to create scalable buttons as Movie Clips in Flash, it could be for you.

This tutorial is for anyone that might need to scale Movie Clips without wanting to lose the shape or affect the integrity of the design. Maybe you need to create a Movie Clip which you might need to resize. Maybe your swf scales to fullscreen and you're sick of the corners getting all distorted. Or maybe you're just a flash addict like me who wants to use any possible feature that might make life a little bit easier. Regardless of the reason, understanding this feature cant hurt.

1. Draw a shape on the stage that you want to use as the basis for your Movie Clip.
   (for the purpose of this tutorial I've set the corner radius to 20 to emphasize this feature)
   
   

2. Press F8, or select Modify>Convert to Symbol... to turn you shape into a symbol.
   
   
   
3. Select the bottom checkbox to "enable guides for 9-slice scaling"
   
   
4. Now your symbol in the Library will have dotted guides in the preview window at the top of you library. But here is the catch: to use the 9-silce scaling, you have to edit the symbol from the library, not on the stage.
   
5. Position the guides so that the area you do not want to scale is in the center of the guides.
   
   
   
   
6. Now scale your Movie Clip directly on the stage or using code.
   Note: If you use the Transform tool to scale your symbol on the stage, the 9-slice scaling will not take effect until runtime.
   OR
   
   
   

7. Now publish your movie and see what you have really done with 9-slice scaling.