Our galvanised arris rail morticed posts are designed to fix wooden arris rails to concrete or timber posts. The bracket fixes to the pre-formed slot of the concrete post and the arris rail is then mixed onto the bracket. The aOur galvanised arris rail morticed posts are designed to fix wooden arris rails to concrete or timber posts. The bracket fixes to the pre-formed slot of the concrete post and the arris rail is then mixed onto the bracket. The arris rail can be secured without disturbing the post in the ground.rris rail can be secured without disturbing the post in the ground.