Answer:

Aestivation is the arrangement inside a bud of the floral parts until it blooms into a flower. Vexillary aestivation is the unique arrangement where the other smaller petals are covered by one large petal. The larger petal is called a vexillum, while the two curved petals which are laterally positioned are called wings. In this destination, the two inmost, boat-shaped petals are called keels.
Examples: Polypetalous, papilionaceous.