X:ID is used to identify a previously unidentified object from a list of objects based on criteria provided by an author and selected by a user. The user selects these criteria from a list of possibilities and these selections reduce the list off possible objects down until the number of possibilities is one.

The object list is often a group of organisms but may be anything from automobiles to medical conditions to mechanical repairs. We developed X:ID for use with biological taxonomic keys so our examples will use these

X:ID provides the interface for a user to diagnose or identify a single object from a list of objects. This list is the taxa list. When the key is first loaded, all taxa in the list are possible matches to a users inquiry. As criteria are selected by the user, some taxa are discarded and while others remain. A successful keying session occurs when the taxa remaining equals one and all other taxa in the list have been discarded.

The criteria that a user selects to make the distinction between taxa to discard and those that are relevant are divided into character lists. For example, a key to insects might use the presence or absence of wings or the type of mouthparts to distinguish between different groups of insects. These major divisions are referred to as characters and each character in a list is further distinguished by one or more states.

States are the individual choices within a character that a user selects. For example, the Character "Mouthpart Shape" might have the following possible states

• "No visible mouthparts"
  
• "Large pincer-like mandibles"
  
• "Coiled tube"
  
• "Straw-like or tube-shaped beak"