Doubles

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. (from American Daylily Society)