   Depth vs Depth Stencil buffer
  With DirectX, you have all these different formats for the Depth buffer. I fully understand z buffering but what I'm not clear on is why you would want to use a depth/stencil buffer instead of just a plain depth buffer?Is the stencil part not being used? and if so, is there some effect that going on?For example, see the below flag...D3DFMT_D24S824 bits of depth and 8 bits of stencil. What is that 8 bits doing?   
  It's being used as the stencil buffer. The depth buffer and the stencil buffer are often allocated and described together, but they are two different things. Googling for "stencil buffer" should turn up plenty of information.   