[Roman]

I need Tips on coverage.
What little I know (I hope):
- Trees and rocks are good for cover. Obvious that the slopes of the hills. Orchards and sown fields are also effective.
I do not know:
- The coverage is more efficient if a unit has several turns in the ground covered?
- What is the best coverage for the tanks?
- What would be the order of the terrain capability of cover?

I searched in the search and found nothing specific.
Thanks.