A postmark is an official mark stamped on mail by the post office showing when and where a letter or package was sent. You'll see it printed in ink right over the stamp, often with the date, time, and city name. The postmark proves the item went through the mail system and cancels the stamp so nobody can peel it off and use it again.

Before email and texting, postmarks mattered a lot. If you mailed your contest entry or tax forms by the deadline, the postmark date proved you sent it on time, even if it arrived days later. Today, people collect old letters partly because the postmarks tell stories: where someone traveled, when they wrote during wartime, or what small towns existed long ago.

The circular or rectangular mark usually smudges part of the stamp, and collectors of rare stamps sometimes prefer ones without postmarks, called mint condition stamps. But historians and other collectors prize postmarks themselves as little pieces of evidence showing how mail moved through history. A postmark from your great-grandmother's wedding announcement connects you to that exact moment when she dropped her letter into the mailbox.