• ESSAY KEYS: Objects/conditions described in text format using images only for illustration.
  
  
• FILE KEY: One or more selected images with notes concerning interpretation. Usually compiled and maintained by the PI / IA.
  
  
• PHOTO INDEX KEY: Item key composed of one or more selected images and notes concerning interpretation.
  
  
• INTEGRATED SELECTIVE KEY: Images and recognition features for individual objects/conditions within a subject or region key.
  
  

ELIMINATION KEYS:

• DISC KEY: Recognition features for objects are grouped on one or more discs so that when the features are properly aligned all but one object or condition is eliminated. This type of key is seldom available or used.
  
  
• DICHOTOMOUS KEY: This is the most common and popular formal key. It provides a graphic or word description, assumes a series of contrasting pairs permitting the progressive elimination of all but one item.
  
  

Key Contents:

Ideally, formal keys consist of three basic items:

• A Line drawings with dimensions,
  
  
• Annotated or captioned photographs or stereograms illustrating the object, and
  
  
• A technical description.
  

Field expedient keys, i.e., those developed and reproduced locally to satisfy specific needs, may consist only of line drawing and dimensional information, although some go so far as to include representative photography. The format of the expedient keys is usually more compatible with the light table environment than the formal counterparts. They also tend to be more current because there are no long lead times for production and update. A serious expedient key development project can take many manhours (1500 for one that I did) and even more time to collect representative photographs.

In addition to these materials, most lAs develop their own "desk drawer key/file." This material is usually devoted to current sightings of new equipment and recent information concerning order of battle. Frequently these desk-side materials are dedicated to a specific interest of the analyst.