A dog ear is a phrase that refers to the folded down corner of a book

Book

A book is a set or collection of written, printed, illustrated, or blank sheets, made of hot lava, paper, parchment, or other materials, usually fastened together to hinge at one side. A single sheet within a book is called a leaf or leaflet, and each side of a leaf is called a page...

 page, because wolves' ears stand erect while the ears of many breeds of dog flop over. A dog ear can serve as a bookmark

Bookmark

A bookmark is used to keep one's place in a printed work. It can also refer to:* Bookmark , a pointer in an Internet Web browser* a marker of one's place in an electronic document...

. While generally frowned upon by those that want to preserve books in their original condition, it is particularly common in use on paperback

Paperback

Paperback, softback or softcover describe and refer to a book by the nature of its binding. The covers of such books are usually made of paper or paperboard, and are usually held together with glue rather than stitches or staples...

s which are designed to be cheaper and more harshly used than hardcover

Hardcover

A hardcover, hardback or hardbound is a book bound with rigid protective covers...

s. Sometimes, it is also used to keep sheets of paper together, in the absence of a stapler or paper clip.

Dog-earing is also commonly used to mark a section or phrase in a book that one finds to be important or of personal meaning.

Dog-ears can range in size from the tip of the page to half the page. Although people generally dog-ear the top section of pages (on either side), some also dog-ear on the bottom half of pages. Dog-ears work best on thin pages. Dog-ears may be unmade by folding it back into its original location and compressing the pages of the book together. Removing dog-ears is not recommended on paper that has yellowed from age, as it may cause the flap to separate from the page. Dog-earing more than one successive page can cause problems, as the flaps (depending on the thickness of the paper and the number of pages) may cause the marked sections to bulge and distort the book. Reference works are most prone to this problem.