Format
Blocks of stamps from the edges of the original sheet or pane often include sections of the sheet's margin, which may have a wide variety of information. For instance, arrow blocks preserve the guide lines used by line up the sheets for perforation or other production steps (these are usually angled in an arrow shape, thus the name), and center line blocks includes lines printed down the middle of a sheet. An imprint block includes the name of the printer, while for many United States stamps the zip block includes a promotional mention of the ZIP code.
Read more about this topic: Postage Stamp Block