The gutter strip consists of 10 stamps from the Universal Postal Union – 150 Years stamp issue, five on either side of the blank strip separating the two panes of a stamp sheet.

The Universal Postal Union (UPU) facilitates a uniform, worldwide postal system. It sets the rules and standards for the exchange of international mail among its 192 member countries. An agency of the United Nations, the UPU is based in Bern, Switzerland.

The UPU was established 150 years ago, on 9 October 1874, initially as the General Postal Union. Before 1874, any two countries that exchanged mail had to negotiate a postal treaty. If there was no treaty providing for direct delivery of letters, mail had to go through an intermediate country, a process both unwieldy and time consuming. The UPU effectively created a single postal territory and established a flat rate to mail a letter anywhere in the world. Stamps from originating nations were to be accepted along the entire route, including transit countries, with the originating country keeping the postal revenue. It also determined that postal authorities give equal treatment to foreign and domestic mail. In 1897, UPU members agreed on a streamlined system whereby green (standard printed matter), red (postcard) and blue (letter) were the standard colours for stamps, a convention that lasted until 1953.

Six Australian colonies joined the UPU in 1891 as part of a single entity, “Australasia”; the federated nation of Australia joined on 10 January 1907. Today, the federal government department responsible for communications, working closely with Australia Post, represents Australia at the UPU. Australia is also a member of the Asian-Pacific Postal Union, which facilitates cooperation in the Asia Pacific region.

World Post Day is celebrated each year on 9 October, the anniversary of the establishment of the Universal Postal Union in 1874. The UPU’s theme for the 2024 anniversary is “150 years of enabling communication and empowering peoples across nations”.