On May 11, 5:18*pm, mike wrote:
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?

Depends on the size of the rails, the size and weight of the fence
boards/pickets, the distance between the posts, the material of the
rails (vinyl over steel or aluminum, wood species), the aesethetic
sense of the the owner/installer, etc.

R