1. The key should be strictly dichotomous throughout.
2. The statements in the two members of a pair (couplet) should be contrasting so that one fits the situation at hand while the other does not. For example:
a. Leaves opposite...................
a. Leaves alternate..................
3. The statements are usually in phrase form and are uniformly separated from one another by a semicolon. For example:
a. Leaves opposite; flowers blue; sepals 4.........
a. Leaves alternate; flowers red; sepals 5.........
4. Attempt to phrase leads to read as positive statements, especially the initial lead of the couplet. Note that the first example is more specific than the second:
a. Inflorescence an umbel............
a. Inflorescence a panicle...........a. Inflorescence an umbel............
b. Inflorescence not an umbel........
5. To avoid unnecessary confusion the initial word of each lead of the couplet should be identical. Note that the first example is clearer than the second:
a. Flowers blue; sepals 5............
a. Flowers red; sepals 4.............a. Flowers blue; sepals 5............
a. Corolla red; calyx of 4 sepals....
6. Also, for clarity, two consecutive couplets should not each commence with the same word. Note that the couplets of the first example are more sharply defined than those of the second:
a. Petals blue.......................
a. Petals red........................
b. Corolla actinomorphic.........
b. Corolla zygomorphic...........a. Flowers blue......................
a. Flowers red.......................
b. Flowers actinomorphic.........
b. Flowers zygomorphic...........
7. Avoid the use of generalities or relative terms in contrasting leads of the couplet. Note how much more specific the first example is than the second:
(1) a. Petioles 2-3 cm. long; flowers 5-8 cm. in diameter........
a. Petioles 6-7 cm. long; flowers 0.5-1 cm. in diameter......(2) a. Petioles short; flowers large and showy...................
a. Petioles long; flowers smaller and less conspicuous.......
8. Avoid using overlapping limits of variation. The following example is in poor form:
a. Petioles 2-6 cm. long; flowers pink to red......
a. Petioles 3-8 cm. long; flowers red to purple....Here, petiole length and flower color should be avoided. Other, more contrasting characters should be sought and embodied in the key.
9. Use clearly discernible morphological characters whenever possible. Avoid using cytological data (chromosome number, etc.). Also avoid separation based solely upon habitat or geographical distribution.
10. Keys to dioecious plants should take into account both staminate and pistillate material. Often it is useful to provide two keys - one for each sex (as in Salix).
11. Use constant vegetative characters whenever possible. If vegetative parts do not display diagnostic features use both flowers and fruit. It may be necessary to devise two keys - one for flowering material, the other for fruiting material. Within a single key, avoid switching from flower characters to fruit characters unless fruit and flowers are commonly present together - or unless sufficient supplemental characters are given to identify material in either condition.
