Working through a key separates plants with and without keys characters and gradually narrows down the number of possibilities. Eventually, if all goes well, the number of possibilities is reduced to one and the specimen is identified. All other species This is an indented key and probably the most common type used in floras. Each pair of mutually exclusive sentences is printed in the same column, beginning with the same word. When the pair is far apart on the same page or on different pages a letter is used in front of each statement so that both members of a pair are easily recognized.
Indentation continues with each successive sentence until the end point is reached.
Another type of key uses the same principle, of pairs of statements, but they are arranged differently. At the end of each statement is another number which indicates the next pair of statements to be considered. This type is of key is used particularly when long descriptions or pictures are included in the key. Sometimes both sorts of keys are combined. Keys in which only one character at a time is considered are called monothetic.