Dichotomous keys also require knowledge of terminology, since 

many terms are more precise than using ordinary words

•

With flowering plants, the starting point is usually 
determining if your plant is a 

monocot 

or 

dicot

•

Those two major categories are based on several traits, but 
the terms themselves refer to the number of seedling 
leaves—two for dicots, one for monocots

•

This trait is impractical to use in most cases, since plants lose 
their seedling leaves soon after germinating

•

There are also exceptions to the number of cotyledons, as 
there also are for most criteria to recognize these two groups