Most cancelled, or used, 20th-century U.S. stamps are worth less than the paper they're printed on. But used stamps are priceless in the way they tell the story of America, through images of its presidents, technological achievements, prominent citizens, and natural wonders. These cancellations, which are called killers among collectors, often tell stories of their own, too, pinpointing a letter, postcard, or parcel in a particular time and place.

Cancellations range from machine-canceled stamps to Railway Post Office, or RPO, cancels, which are often collected by those interested in railroadiana. A bullseye cancel, also known as a SOTN, which stands for ‘socked on the nose’, means the entire postmark is readable on the stamp, allowing collectors to see the time, date, and location where the stamp was used.

First day issues and covers usually feature fancy cancels, but these stamps are not considered used, per se, since they were never intended to be mailed. Instead, the cancellation simply serves to verify that the cover was purchased on the stamp’s first day of issue...