MULTIPLE PETAL FORMS. “In core eudicots this ultimately results in three petal types: a medially positioned abaxial (ventral) petal, two lateral petals and two adaxial (dorsal) petals. Each of these separate groups (two dorsal petals, two lateral petals, and single ventral petal) can vary in multiple morphological characteristics to result in a floral form with a single dorsoventral plane of symmetry.”
http://onlinelibrary.wiley.com/doi/10.1111/nph.13198/full