Single flowers consist of two whorls (layers) of perianth segments: the sepals and the petals. A double flower has more than one petal whorl (hose-in-hose double), or a stamen whorl that contains petaloid (petal-like) stamens (peony type double). In the peony type, carpels may also be petaloid. Individual flowers may be a combination of both double types.
Outgrowths of extra tissue from the midrib or elsewhere on a petal are not considered to constitute a double. Extra segments in the normal two perianth whorls classify a flower as polymerous rather than double.