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I googled the question and found no explanation. It seems that dog ears are inevitable (for paperbacks, notably) even if you've always been careful. From my experience, they are about equally likely to appear on the top corners as on the bottom corners (for both the beginning pages and the ending ones). Dog ears for the middle pages of the book are less likely but they can also appear in frequently used old books. Can someone explain why?

dog ears on the bottom corners for the beginning and ending pages

dog ears for the beginning pages of a book

I apologize if this is not the right kind of question to post here. I can find no other sites on SE for it.

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I think moisture does it. It can make the paper curl. Also the fact that you turn pages by using the corners. If you've ever soaked a book in the rain (My copy of Irodov got soaked once :/ ), then you'll notice that pretty much the whole book curls up. THough I dunno if this is the cause. –  Manishearth Apr 10 '12 at 3:29
    
I don't think they have anything to do with turning the page by its corner, because all four corners have them, although perhaps this habit will exacerbate existing ones on the lower front corner (but then again, are there really people who turn pages by using the corners?). –  Eric Apr 10 '12 at 3:49
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Well, the edges. You curl the edges, but the corners have more freedom so the curling of the edges affects them a bit more. –  Manishearth Apr 10 '12 at 3:51
    
This is inconsistent with the fact that they always curl in different directions for the front pages of the book and for the back pages of the book. –  Eric Apr 10 '12 at 4:12
    
Well, I'm not sure. It was just the first thought I'd had.. Moisture and turning of pages. And heat as well. But I don't know how to cohesively merge these into an explanation, since I'm not too sure. –  Manishearth Apr 10 '12 at 4:24

2 Answers 2

Alright, your picture made me understand what you're talking about.

Its the moisture that's doing this. Paper is hygroscopic--it absorbs water from the atmosphere, if only a little bit.

Now, paper is wood (pulp). And wood contains plant cells. Plant cells(or whatever's left of 'em) absorb this water and swell. This causes the paper to "warp" (you may have noticed this while using watercolors--or just dunk a scrap of paper in water and see what happens when it dries). This warping is due to the fact that the paper has limited area, so the molecules have nowhere to expand but up/down.

Now, if you have a bunch of sheets of paper, the warping will not be the same for each one--as in the warped "humps" will not necessarily fit into the warped "troughs" and vice versa. Since the sheets no longer have a snug fit, we have a lot of extra space. This causes the book to "puff up". The corners have more freedom than the rest of the page, so they can at least curl away when puffing up.

I once got a book (physics book, incidentally :/ ) wet in the rain when I'd kept it in the non-waterproof section of my bag. It puffed up like crazy. After drying it and flattening it, it's no longer puffed up as much, but the edge of the book is still pretty U-shaped.

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But a book that's not used will have none of those dog ears at all, although it too absorbs moisture as the used ones. –  Eric Apr 10 '12 at 6:15

I think this is primarily about plastic (non-reversible) deformation. Plastic deformation appears when the stress of material is large enough. Stress is a generally speaking ratio between "force" and "dimension" of the object. At the corners, this ratio is larger, even for the same force, as "dimension" is smaller, that is paper tends to get narrower toward the corner.

Imagine that you want to fold paper through the center or at the very edge. A much larger force will be needed in the former case as the dimension of the fold is much larger.

Part of the problem might be also, that you are usually turning pages by pulling page by its corners, so you usually apply force close to future dog ears.

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