Most flower families can be placed in one of three groups, based on symmetry, or lack of it, in the flowers. Some flowers are radially symmetrical. In other words, the flowers look the same when rotated, just as a wheel looks the same when turned on its axle. Other flowers are bilaterally symmetrical, the left side being a mirror image of the right, just like a face. In still other flowers there is no obvious symmetry, either because the flowers are tiny and inconspicuous, often lacking petals, or because the flower parts are arranged very irregularly. A few families contain both radially and bilaterally symmetrical species. In that case, the family is placed in the symmetry group most common to the family. Thus, snapdragons, usually bilaterally symmetrical, are classified in that group. But the speedwells, members of the Veronica genus within the snapdragon family, and more or less radially symmetrical, are mentioned in the list of radially symmetrical families so that the snapdragon family can be searched when a more or less radially symmetrical four-petaled flower is to be identified.

Radially symmetrical flowers are then further subdivided by the number of petals usual in each family. (In most families the number of petals is constant, the most notable exceptions being the composite, buttercup and poppy families.) The most characteristic features of each family are given in each petal number section. Thus, a radially symmetrical flower with paired opposite leaves and five petals, each notched at the tip, is likely a member of the pink family. If the flowers are tiny and arranged in umbels (umbrella-like clusters), it is likely a parsley family member. If flowers are tiny with four petals, and the stem is square, the bedstraw family should be searched. Similar clues to family identification are provided in the list of bilaterally symmetrical families. For example, if the stem is square the flower is probably a mint.