I notice the old fences around all have 2 horizontal rails for the boards to attach to. I've also noticed that new specs often show 3 rails, without comment. I'm thinking that this might be due to the fence boards being much thinner than they used to be.
I like to place my rails long side vertical, so I don't get excessive sagging over time. But I'm thinking that unless I expect the fence to see hard wind loads or other severe attacks (kicks, etc.), 2 rails might be enough.
Any thoughts?