A field guide is a type of reference book, meant to be carried in the field, to aid in the identification of plants and animals. As they are meant to be carried outdoors, they are generally rather small books and not large tomes that would be more convenient in the home or library. They often make use of classification and identification systems called keys by which one may, through observation, determine the exact identity of a specific plant or animal. A field guide may also be intended for the identification of other naturally occurring objects such as gemstones or other minerals and man-made objects such as boats, cars, coins, or antiques.
Early field guides were nothing more than a collection of drawings of plants and animals with descriptions of these subjects. Many early field guides featured engravings of drawings and paintings painstakingly rendered by an artist from direct observation. Before the invention of photography, these field guides were often the only available tool for identifying plants and animals. In the early 21st century, digital field guides have begun to appear, allowing users to access volumes of information over the Internet with tablet computers and smart phones.
The technical depth of a field guide is dependent on the target audience. Some field guides offer generalized classification tools, grouping subjects loosely. More in-depth field guides make use of identification keys to provide exact determinations of identity. Identification keys are made up of a series of statements arranged in such a way as to guide the user through a step by step process. Each step has one or more possible choices, the selection of which directs the user to the next step, continuing until identification is achieved.
While the concept of a field guide was originally conceived as an aid for identifying natural objects, the scope of this type of book gradually became more and more expansive, especially in the middle of the 20th century. Specialized field guides targeting hobbyists and enthusiasts of all types began to appear, and today, field guides for all sorts of things are available. Collectible glass, antique toys, stamps, and wines are just a few examples.
In modern times, field guides for natural objects still often feature drawings in addition to or instead of photographs. Drawings offer a degree of detail and exposition often not captured by a photograph. Field guides for other objects tend to rely more on photographs than illustrations, but this is not an absolute. Modern field guides can be broad in scope or specialized, covering, for example, all the birds of North America or only the flowering plants of an isolated island.