Regarding the division of a flower into symmetrical halves by only one longitudinal plane passing through the axis: Many flowers are symmetrical in only one plane (that is, symmetry is bilateral) and are termed to be irregular or zygomorphic (meaning yoke-formed or pair-formed).

In irregular flowers, other floral parts may be modified from the regular form, but the petals show the greatest deviation from radial symmetry.

Examples of zygomorphic flowers may be seen in orchids and members of the pea family.