A basic way to organize postcards is by people, places, and things. In the 'people' category, postcards with photos, drawings, or paintings of presidents, movie stars, rodeo riders, or circus clowns can show the variety of ways these notable personalities were depicted for their audiences. Postcards of the works of specific artists (including postcard artists) can show the breadth and evolution of their work. Because so many postcards were produced for the travel industry, gathering cards for specific 'places' or regions can show the character, diversity, or changes in architecture, settlement, ecology, or land use over time. There are well over 500 categories of cities, states, and countries currently in widespread use. 'Things' of interest include topics like cats, mermaids, trains, diners, lighthouses, western art, holidays, and humor. These are often called 'topical' cards and frequently display the trends and interests of an era.