The Nuts and Bolts of Classification

Introduction and Application


          Classification systems are used constantly in everyday life. In a grocery store, for example, items are classified into food or non-food items; food items into perishable or non-perishable; perishable into meats, produce, dairy or frozen; meats into chicken, beef, pork, lamb or fish; produce into fruits or vegetables; etc. When you go for a walk outside you automatically classify things you see (examples: trees, flowers, birds, insects, etc.). Classification by using a dichotomous key is a way of identifying distinct traits specific to one individual object or organism.

          In science, as well as in everyday life, people attempt to organize objects, ideas or information. Placing objects, ideas or information into groups on the basis of similarity is called classification. We classify things in order to locate them more easily, see how they are similar or different, find order in the world and so we can communicate with each other by knowing we are talking about the same thing.

          A classification key is a tool which organizes the traits of a particular group of organisms so that these traits may be used to identify a particular organism in the group. One type of classification device is a dichotomous key. Dichotomous keys have a series of numbered steps which take the user from very general to more specific traits. Each step usually consists of two statements (there are exceptional times when ther may be more than two choices) about a single trait. Only one of these statements can be true about a single organism or object. The key helps the user narrow his choices until he arrives at the step which identifies the organism or object. Each key is specific for a group of organisms or objects. For example, you may have a key for wildflowers in a particular

region, trees in a particular region, freshwater fish, insects, etc. A key can only be used to identify organisms or objects that are present in that key.

The process of using a dichotomous key is the same from key to key even though they identify different organisms or objects. Begin by observing the organism or object in question and noting the general characteristics.

Using the key, read both statements in the first step. Decide which of the statements is true. This will lead you to another step in the key. Continue to read the statements in the steps to which your choices lead you until you arrive at the name of the organism.


In this particular exercise you are going to create your own classification key for the objects provided.







Materials and Methods:


1. Students will be arranged into small groups of 2-4 students per group.

2. Each group will receive the following materials:  different pieces of

    hardware, rulers.

3. Instructions:

          a. Remove all the hardware pieces from the plastic bag and observe

            some general traits.

          b. Divide the objects into at least two groups based on the presence

             or absence of one trait. (Example: Objects with threads and

             objects without threads.)

          c. Place each object into the proper group.

          d. Write the statements at the bottom of a piece of paper, one to the

             left side and one to the right. (Keep in mind it is possible to have

             more than two statements).

          e. Draw 2 diverging lines above each statement on the paper. (See


          f. Observe the objects in one group at a time. Further divide the

            objects based on a single trait for each group. Write the  

            statements above each group and place the objects in the proper

            group. (Example: In the group of objects without threads, you may

            observe that some have heads and some do not.)

          g. Continue to divide objects in each group into two subgroups until  

            you arrive at the level where there is only one object in each  


          h. At the final level, draw or trace each object next to the

             appropriate statement.

          i. Place all hardware back into the bags and write the names of all

            the group members on the back of your papers.

Sample Diagram



                                with    without

                                 heads    heads

                                objects with                                    objects without

                                      threads                                               threads


The following diagrams and terms are designed to provide some clues to your investigation.


left to right: Phillips head screw top , round-head machine or metal screw, flat-head wood screw, and round-head wood screw

Source: Academy Artworks and


Binomial Nomenclature: A two-name system of naming organisms.

Classify: To put objects, ideas or organisms into groups based on


Dichotomous: Divided into two parts; branched or forked into two more or

     less equal divisions.

Genus: A group of similar species.

Organism: Any living thing.

Scientific Name: The two-part name of an organism consisting of the    

     genus and species.

Species: A group of closely related organisms capable of mating and

     producing viable offspring.

Trait: An inherited characteristic.