In philately

Philately

Philately is the study of stamps and postal history and other related items. Philately involves more than just stamp collecting, which does not necessarily involve the study of stamps. It is possible to be a philatelist without owning any stamps...

 a backstamp is a postmark

Postmark

thumb|USS TexasA postmark is a postal marking made on a letter, package, postcard or the like indicating the date and time that the item was delivered into the care of the postal service...

 on the back of a letter

Cover (philately)

In philately, the term cover pertains to the outside of an envelope or package with an address, typically with postage stamps that have been cancelled and is a term generally used among stamp and postal history collectors. The term does not include the contents of the letter or package, although...

 showing a post office or station through which the item passed in transit. The office of delivery may also backstamp a cover and this type of mark is known as a receiving mark.

Backstamps are often applied as documentation of transit times, lengthy ones in the case of ocean crossings or short ones in the case of airmail

Airmail

Airmail is mail that is transported by aircraft. It typically arrives more quickly than surface mail, and usually costs more to send...

 flights. Registered mail

Registered mail

Registered mail describes letters, packets or other postal documents considered valuable and need a chain of custody that provides more control than regular mail. The posted item has its details recorded in a register to enable its location to be tracked, sometimes with added insurance to cover loss...

 is often backstamped in order to show the chain of custody

Chain of custody

Chain of custody refers to the chronological documentation or paper trail, showing the seizure, custody, control, transfer, analysis, and disposition of evidence, physical or electronic...

.

Mail that has had complex routings can have a dozen or more backstamps. Although such covers may look positively blackened with the overlapping marks, they are not common and so are highly valued by collectors of postal history

Postal history

Postal history is the study of postal systems and how they operate and, or, the study of postage stamps and covers and associated material illustrating historical episodes of postal systems...

, being described as "well travelled".