In a residential situation a letter plate would probably have an opening size which, although of sufficient size, does not give intruders the possibility of inserting a hand to operate a lock snib turn or door bolt. The common size for a domestic front door would be 10” x 3”. However, a commercial building may need to accommodate large bulky envelopes, so a letter plate which is 16” x 5”, or indeed, a separate letter box fixed to a wall may be more suitable.
Letter plates are usually fitted horizontally, but vertical types with small apertures are available. Postal knockers have an additional hinged/pivoted knocker to double up as a letter plate and knocker combined. Letter plates for horizontal use are provided with outward opening gravity flaps or inward opening spring flaps. Letter plate tidies or interior flaps are used internally to provide a draught proof cover or flap. Popular finishes include brass, PVD brass, chrome, stainless steel, bronze and aluminium.