x1 is a side of sheet/leaf of paper/page/thin writing surface (card), or is a quantity of text equivalent thereto, of sheet/leaf of 'paper' x2 in document/book/bound mass or sequence of pages x3.
In physical books or the like, pieces of paper (x2) are what is touched, flipped, or held, but sides of paper/pages are what are numbered (2 per piece of paper) and what are read. See also: "papri".