[From Peonies, by Jane Fearnley-Whittingstall, p. 208:] In China today there are nine recognized flower shapes for peonies; they can be translated as single-petal, lotus, chrysanthemum, rose, sweet-scented osmanthus-holding, golden-ring, crown, silk-ball and pavilion... [four categories are] recognized by the Royal Hotricultural Society: single, semi-double, double and Japanese (also called imperial or anemone form).
[From Peonies, by Allan Rogers, p. 8: single peonies have] a shorter vase life perhaps, compared to their more popular double counterparts, but [are] still well worth the effort...