Armour skills and what you're wearing

How much of a type of armour do you need to wear in order to have access to the skills in the tree for that type?

Examples include, wearing all cloth but a plate helm - is that enough to give me access to the skills in the heavy armour tree?

If I'm wearing plate on my chest and legs, a chain coif, leather gloves and boots - can I access all three types?

I don't see why this shouldn't be the case, since I would still need to spend points on those skills, but I think this should be thought about carefully.

 

Right now, it seems that skills only activate based on what chest piece you wear. Mixing and matching probably won't ever be optimal, but perhaps activating both on chest and legs would be nice, to get benefits from different skill trees.