For my photography final project, we're required to frame and mat our prints. I've decided to print at 6x9, and mount on an 11x14 so it'd fit into a portfolio case (11x14). While I was on FrameDestination.com, under the Bainbridge 4Ply Alphamat, it gives me an option of "single mat", "double mat" and "triple"; what does it mean? Is it asking me about the shade of the mat? How am I supposed to know what single/ double/ triple mat looks like? Lastly, when it comes to choosing the color of the mat (black/ white), is it advisable to place black on black, or should I go with white on black? Is there a preference in the "artworld"? I apologize for bombarding you with questions; this isn't something that was covered in intermediate photography class today. My professor had a lot trouble with her demo. I have enclosed some sample project photos if that helps any! Thank you!