A popular way of putting works about the same subject in roughly the same place on the shelf. Even if you don't find the book you were looking for, you'll be in the neighborhood of other books on related topics.

The Dewey Decimal System is used to encode the main subject of a work. Other subjects, no matter how important they may be, are not usually represented in the Dewey Decimal System number.

Dewey Decimal trivia: 999 stands for "extraterrestrial worlds".