First of all, you need to examine the objects you will be comparing. Characteristics used in the dichotomous key must be constant or, in other words, not vary. The user of your key should be easily able to identify the answer to the questions. Therefore, make sure you use the correct terms and, when necessary, use measurements rather than objective words. Here's how to create a dichotomous key.
In this key, we will be comparing the desk items of pencil with eraser, pen, paper clip, staple, thumb tack, and eraser, as everyone is familiar with these items and no technical terms are necessary.
1a. Object is used for writing.......... Go to 2.
1b. Object is not used for writing.......Go to 3.
A clear division between our items are items used for writing and other desktop items. This is a good comparison as there will be no confusion about where the object fits and the items will be clearly divided.
2a. Object contains ink........Pen
2b. Object does not contain ink.......Pencil with eraser
Our writing objects only had two objects remaining - pen and pencil with eraser. This is a good time to split them in two, and determine the object. The most obvious difference is compared here.
3a. Object is used to attach papers......Go to 4.
3b. Object is not used to attach papers......Eraser
Being left with eraser, thumb tack, paper clip, and staple, we once again need to divide our objects. Here we classify the paper clip, thumb tack, and staple together, while leaving eraser to be revealed.
4a. Object does make holes in paper...........Go to 5.
4b. Object does not make holes in paper.......Paperclip
Both the thumb tack and staple make holes in the paper, leading them to be categorized together. The comparison here can be easily made to reveal the paperclip.
5a. Object attaches paper to paper.......staple
5b. Object attaches paper to hard surfaces.....thumb tack
Left with the staple and thumb tack, the obvious comparison is the surface of what the object is used to attach paper to. All objects have now been revealed, and the dichotomous key is complete.