Size. Letter sized is 8.5" x 11" and is commonly used in the U.S. and much of North America. A4 is a slightly different size (can't remember the dimensions at the moment), and is usually used in Europe and (I believe) Asia. My local Staples has both sizes in 110lb cardstock on the shelves, and it should be on their website, too.I know the files were posted awhile ago...but I just saw this thread through a link you posted...and I just wanted to check and make sure.....a4 and letter have to do with the type of cardstock right? Just making sure so I download the right one.