This triangle-shaped stamp has an unusual purpose

A new U.S. postage stamp is triangle-shaped, and it’s valid on mail sent around the globe to more than 180 countries.

The triangle Postcrossing stamp from the U.S. Postal Service commemorates an international pen pal project started in 2005 by Paulo Magalhães, a student in Portugal. The program connects people around the world in a simple but increasingly old-fashioned way: Send a postcard, get one back.

What started as a website Magalhães hosted on his personal computer has since spread around the world. Today, more than 805,000 people from more than 200 countries and territories have sent more than 80 million postcards through the program. Americans have sent more postcards through Postcrossing than the residents of any country except Germany, but maybe a new stamp could put the U.S. on top.

Postcrossing has gotten its own stamps before in countries including Germany and Finland. The U.S. Postcrossing stamps come following a letter-writing campaign by supporters to the USPS Citizens’ Stamp Advisory Committee, which recommends future stamps. Their successful letter-writing campaign also marks the return of a rare postage stamp shape.