The Gospel of Paper Grain
So while at the aforementioned calligrapher, I mentioned that I was helping folks hack up little long-stitch signature-bound books. I explained that it was really hackish, and not for raw aesthetics.
She immediately pegged me with how the grain worked out in our little laser-printed setups. I kind of stumbled for a moment, and she showed me a few demonstrations about how paper grain affects binding.
Basically, machine-made paper is created in a roller, and the fiber particles get stretched in one direction. Typically on standard paper this follows the top edge, so that when you read an ordinary loose-leaf sheet, the grain runs to the right and left.
She handed me a book that had been bound incorrectly, with the grain running perpendicular to the spine. It was noticeably difficult to turn pages and keep them flat. I would have criticized the paper as being "too stiff" or perhaps badly glossed, but it was simply a matter that it was bound in the wrong orientation.
Take a ream of ordinary letter paper (or A4 I guess), and lay a heavy book two or three inches from the top edge. Try flipping through that top edge. Now do the same with the right edge. You should find that it's tougher to flip that way, even if you cut the ream into a perfect square.
Likewise, notice that when you fold or tear a piece of letter paper, the crease or tear is straighter when it follows the grain. It's no accident then that the standard American letter-paper 'zine format has as its spine the 8.5" bisecting line of an 8.5"x11" piece of letter paper.
Another test you can do is to dampen a piece of paper and watch it dry. Maybe you have a phone book that was under the sink for too long, or a favorite toilet reader that spent too much time in the shower steam. Maybe you spilled coffee on a magazine or textbook. You should see that the ripples of the distorted paper are all visible mostly from the top and bottom edge of the monograph, while the foredge and spine are still mostly straight.
This is because the fibers in the paper like to stretch out when they're waterlogged, creating uneven tensions in the structure of the sheet. The importance of this fact is that when you bind with glue along the spine (typically for perfect-binding, though some signature binding techniques have been known to do the belt-and-suspenders thing), you don't want the spine itself to try and lengthen, as that will crack the pages out when the ripples start forming.
I saw this happen many times with the books that the Internet Archive Bookmobile were handing out, and it wasn't until I got religion on paper grain that I realized what had caused it. It's a real pity that they were handing out such poorly-bound books. Had they made sure to make the spine parallel to the grain, the books would have been easier to manipulate and more durable besides.