If the largest petal of corolla at the posterior end known as vexillum overlaps the two lateral petals known as wings or alae,  the wings or alae in return overlap the posterior small, fused keel or carina petals and the overlapping starts at the posterior side and advances towards the anterior side, the aestivation is described as

Valvate aestivation - adjacent sepals or petals of a whorl touch each other but do not overlap. ex. Calotropis, calyx of pea, makoi,Twisted aestivation - each sepal or petal of respective whorl has one margin overlapped and the other one is underlapped. ex. corolla of lady's finger, cotton, china-rose, datura.Imbricate aestivation - margins of sepals or petals overlap one another but not in a particular direction. eg. corolla of caesalpiniaceae members like Cassia, gulmohurVexillary or papilionaceous aestivation - the largest petal at the posterior end known as vexillum overlaps the two lateral petals known as wings or alae. Wings or alae in return overlap the posterior small, fused keel or carina petals. The overlapping starts at the posterior side and advances towards the anterior side. ex. Fabaceae members like pea and bean. it is the feature of fabaceae.