| Summary: | To enable the field identification of trees, keys of different nature are used, usually based on vegetative characters. These keys may be of three types: dichotomous (one dimension), tablet (two dimensions), and sets of punched cards (three dimensions). A general description of the three types is presented. Special emphasis is given to the punched cards, their multiple use, and their preparation. The most important papers on punched cards keys in plant taxonomical research are reviewed. The problems commonly faced in searching for and selecting suitable sexual vegetative characters to be used in the cards, specially in connection to the tropics, are discussed as well as the advantages and disadvantages of the punched cards in comparison to dichotomous keys.
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