<P> Constructed of light - gauge painted sheet steel , Joroleman designed his mailbox with an arched , tunnel - shape roof , which prevented excessive accumulation of rainwater or snow while resisting deformation . The tunnel top also simplified the process of mass production by eliminating the need for precise sheet metal bends . Stamped and formed metal straps riveted to the arched opening and the mailbox door served as a door latching mechanism , while a rotating red semaphore flag mounted on a shaft attached to the side of the mailbox served to signal the approaching mailman if there was outgoing mail inside . Fitted with a crimped or braze - on rear steel panel and a false floor to keep its contents dry in inclement or humid weather , the Joroleman mailbox required only two rivets , three axle bolts , and four screws and nuts for completion . Durable and inexpensive , the popularity of the Joroleman mailbox was further enhanced by a decision not to patent the design , but to make its specifications known to all potential manufacturers for competitive sale . Adopted across the United States , it has remained the top - selling mailbox since its introduction , and was also widely used in Canada prior to that country 's decision to eliminate individual curbside delivery to rural residents . </P>
what is the red thing on mailboxes for
to signal the approaching mailman if there was outgoing mail inside