The postmark actually predates postage stamps. It was used to show the time and place when a letter was sent. Once postage stamps were introduced, the postmark was also used to "cancel" stamps, or show that they had been used (and mark them so they could not be used again).
As postmarks developed many shapes and patterns were used to both surround the date and place, and cancel the stamp. The two functions of a postmark are often set up along an axis so that the postmark can be read clearly on the envelope and the cancellation pattern or mark appears over the stamp(s).
Collectors have varied responses to postmarks and cancellations. Marks that perfectly overlay a valuable stamp which appear clearly are often called "Bullseye" or "Socked-on-the-Nose" cancellations. These are valued for both the information and aesthetic qualities of the marks. Hand-cancelled marks often obscure stamp designs and are less desirable.
Some envelopes, especially international letters, might receive several postmarks to show both the sending and arrival information.
Postmarks began to be used to carry slogans or other information as well as the date and place sometime in the 20th century. US Postal Service marks often carry slogans related to the area or current events at the time.
I've always loved these glimpses into other places and times. As I began this project I first chose stamps with minimal markings, but I am beginning to process and post more stamps with postmarks visible, as well as full original letters with multiple stamps, postmarks and cancellations. Certain marks have an overlay that matches the pattern and design of the stamp beautifully.
