rarely seen except in rural areas. Attached mailboxes are less common in newer urban and suburban developments and in rural areas of the United States, where curbside delivery or delivery to a community mail station (cluster mailbox, known as a bank of post boxes in the UK) is generally used. Rural and some suburban areas of North America may utilize curbside mailboxes, also known as rural mailboxes. These receptacles generally consist of a large metal box mounted on a support designed primarily to receive large quantities of incoming mail, often with an attached flag to signal the presence of outgoing mail to the mail carrier. In the US and Canada, rural curbside mailboxes may be found grouped together at property boundaries or road/driveway intersections, depending upon conditions. Although the United States Postal Service (USPS) has general regulations stating the distance a letter box may be from the road surface, these requirements may be changed by the local postmaster according to local environment and road conditions. At one time, nearly 843,000 rural Canadian residents used rural (curbside) mailboxes for private mail delivery, though Canada Post has since required the installation of community mailbox stations for many rural residents. In the US, wall-mounted or curbside mailboxes that are only designed for receiving incoming mail are known as "limited-service" mailboxes, while mailboxes equipped with a mechanism for notifying the postman to collect outgoing mail from the mailbox are known as "full service" mailboxes. A number of designs of letterboxes and mailboxes have been patented, particularly in the United States. One design was the visible mailbox (because it was made of transparent glass) with a flip-up aluminium lid produced during the first part of the 20th century by George F. Collins of the Barlet-Collins Glass Company in Sapulpa, Oklahoma. Letter box standards and construction Europe The European standard for letter boxes, EN 13724:2002 "Postal services – Apertures of private letter boxes and letter plates – Requirements and test methods", replaces earlier national standards such as BS 2911:1974 "Specification for letter plates" or DIN 32617. It specifies among other things: Envelope size C4 (229 mm × 324 mm) must be deliverable without bending or damage The internal volume must able to hold at least a 40 mm high bundle of C4 envelopes Aperture width of either 230–280 mm (> C4 width) or 325–400 mm (> C4 height) Aperture height of 30–35 mm Mounting height of