The United States Post Office was very concerned in its early years with the reuse of postage stamps. In 1867 when a decent salary was a dollar a day, first class postage represented 3% of an average person’s wage (equivalent today to about $6) so the appeal of reuse was real. Postal historians looking at the record have long questioned whether reuse was a real or phantom problem (my own experience is that it rarely happened) but the fact remains that the Post Office was always looking for schemes to frustrate reuse.
In 1867, they set upon a plan of grilling postage stamp paper. The grill would cut the paper with fine ridges so that any cancellation ink would be absorbed more fully into the paper fibers, making washing of cancellations more difficult. It was a cumbersome solution to an imaginary problem-a perfect government program. But for collectors of United States Stamps it has produced a bonanza of interesting collectible varieties. Check your Scott catalog from #79 to #101. There are many different grill types as experimentation progressed and the grills were impressed on all stamps available at the time. The scarcer grills are actually much rarer than their selling prices would indicate. They are worth learning something about. A good place to get information would be the American Philatelic Research Library (http://www.stamps.org/) . At the same time if you are not a member you should consider joining the APS. Its the best hobby value there is.