December is peak season for the U.S. Postal Service. The average haul for the USPS on a typical day is 523 million pieces of mail, and that increases to 553 million pieces from Thanksgiving to New Year’s Eve. (On the busiest day, December 16, the postal service processes approximately 640 million pieces.) Throughout the holidays, they deliver about 15.5 billion pieces of mail in total [PDF].

After sorting through your end-of-the-year influx of cards, gifts, family newsletters, and catalogs for weird chocolate companies, you may have wondered: What are those marks and numbers inked on top of items sent through the mail? And what do they mean?

The USPS has a collective general term for the stamps, meters, and other items used to process mail: “postage evidencing systems.” The ink markings on the upper-right corner are called “information-based indicia,” and they denote a piece has been paid for and signal where to enter it into the mail flow.

Postal marks—manually-pressed ink markings stamped over a stamp—are the oldest, most basic “indicia” and date back to 1660s England. At their most basic, they show the recipient where and when the piece of mail entered the postal system. In addition, postal marks have the dual purpose of marking a stamp as canceled and used. In complex cases, some show the item’s history of returns and forwards and damage. In the past, some postal agencies have had unique postmarks indicating the item’s method of travel (railroad, stagecoach, ship, etc.). U.S. mail sent with a stamp still bears the city and state from which the mail was sent and the date and time the USPS took custody of the item. It also often features a design, ranging from Lady Liberty to the snowflakes and message of “Happy Holidays” adopted by the USPS for the festive season.

As for those serial numbers on mail sent via an office postage meter, those numbers are vendor codes. They reveal the meter’s manufacturer and model. (There are currently six private contractors licensed to provide meters to the USPS: Data-Pac Mailing Systems, Francotyp-Postalia, Neopost, Pitney Bowes, Endicia, and Stamps.com). The series of numbers beginning with 000 is a serial number for the specific meter.

In one part of the country, people “hack” the USPS’s indicia to make their holiday mail even more festive. Each year, especially holly-jolly denizens from neighboring towns flock to Bethlehem, Pennsylvania, on the eastern edge of the state, to have their holiday mail stamped from Bethlehem.