A flower is typically composed of four series of floral parts. Two whorls of sterile (asexual) parts, plus two whorls of reproductive (sexual) parts: A sepal is a part of the outermost whorl or spiral of a flower. Collectively, the sepals form the calyx. These are sterile parts. Sepals are usually green and foliaceous. A petal is a part of the second series of floral parts (from the outside). Collectively, the petals form the corolla. These are also sterile parts. Petals are often colored and more or less showy. They play a great role in the attraction of pollinators. Petals alternate with sepals. Together, the calyx and the corolla form the perianth ( = "around anthers").

Sometimes it is not possible to differentiate between sepals and petals, when they are very much alike; the term tepal is then used to name each individual part of the perianth, like in most of the Liliaceae (see on photo 04: Gloriosa superba). This example is prairie cinquefoil (Potentilla arguta).