I study dependency grammar, DG and I have a question regarding constraints of DG.
I do understand why do we need constituency grammars, CG and DG, however I don't completely understand the connections between them.
Constraints of DG:
1) every constituent has only one governor.
Very simple constraint, the same we can observe on the tree of CG, there is just no nodes with two parents.
2) dependency grammar forms tree, there is no cycles.
Completely makes sense, the same is in CG.
3) Projectivity. There is no intersections between the arcs of dependency tree.
This is a strange constraint, it doesn't follow from the CG, because the dependency relation we define only on the level of DG, there is no such a thing in CG. And for some reason we should define them so that the arcs don't intersect. But what the reason to do so? And in order to do so we need to change intuitive and simple rules of dependency.
As I understand the DG is not a particular case of CG, we cannot derive DG from CG, just because there is no constant set of rules of defining dependency, so DG contains some new information CG doesn't have. If not projectivity constraint, could we talk about constant set of rules and say the DG is the special case of CF?
In short, what are the connection between DG and CG? What the role of projectivity in DG?