Calyx present: the calyx, the part of the flower which lies below the petals, consists of usually green elements called 'sepals':

The sepals may be free...:

...or fused together at least at the base:

In several species whose flowers are bilaterally symmetrical the calyx becomes bilaterally symmetrical as well, such as in the example below where the calyx is 2-lipped:

Calyx absent: the corolla is not always subtended by a calyx and in this case the flower has only petal-like, brightly coloured elements (usually called 'tepals'):

The involucre of bracts or scales which surrounds the heads of the Asteraceae family should not be confused with a calyx:

In many species of Asteraceae the true calyx is transformed into a series of hairs or bristles called 'pappus', and is here considered as being 'absent':