The library uses the Dewey Decimal System. Its divided into ten basis categories. I divided my pattern subjects into 9 categories (saving the last for something?). They are Southwest, western (cowboy), farm, transportation-occupation, signs and symbols, animals, fantasy, victorian, and Sports & hobbies.
These can be subdivided with unlimited use of numbers (ie. 553.36 is girl scouts logo) 5 is for signs and symbols, 53 is my division for logos and .36 is my 36th logo.