The question was what do clover, sweet pea and vetch plants have in common, apart from being generally leguminous?
They all have flower shapes that are papilionaceous. That is to say they have a butterfly-like form.
The flower head has a large upper petal with a distinct vertical groove, called the banner or standard petal. Two lateral petals are attached to this that look like the wings of a butterfly, hence the 'Papillion' association. Beneath the wings are two petals that have fused together to produce a keel shape. These protect the stamens.
Here we reintroduce the word inflorescence that was used when we were talking daisies. The clover flower, like the daisy, is composed of many little flowers, or florets. If you look closely each individual floret can be seen with its wings and keel.