PowerPoint has a lot of great preset shapes, but sometimes what you really need is a custom shape tailored-made for your presentation. If you’re looking to go beyond the Freeform Tool to create more complex custom shapes, we’ve created a new feature in PowerPoint 2010 called Combine Shapes to help you do just that! Using the principles of Boolean Geometry, the Combine Shapes tool allows you to create new shapes by combining multiple shapes in one of four ways: Union, Combine, Subtract, or Intersect. 
This is a relatively advanced feature, so it doesn’t appear by default on the Ribbon. To enable Combine Shapes, add it to your Ribbon via the PowerPoint Options dialog: PowerPoint Options > Customize Ribbon > Commands Not in the Ribbon > Shape Combine, Intersect, Subtract, Union. (For more information on how to add items and customize your Ribbon and QAT, see this post).
reference:
http://blogs.msdn.com/b/powerpoint/archive/2010/02/01/creating-custom-shapes-in-powerpoint-2010.aspx
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