# How does the post office know if a stamp has been used? **URL:** \ **Category:** Factual Questions\ **Created:** [December 20, 2002, 11:32pm UTC](https://boards.straightdope.com/t/how-does-the-post-office-know-if-a-stamp-has-been-used/144136 "2002-12-20T23:32:04Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![Jayrot](https://avatars.discourse-cdn.com/v4/letter/j/3ec8ea/32.png) [@Jayrot](https://boards.straightdope.com/u/Jayrot)\ **Post date:** [December 20, 2002, 11:32pm UTC](https://boards.straightdope.com/t/how-does-the-post-office-know-if-a-stamp-has-been-used/144136/1 "2002-12-20T23:32:04Z")
[sup]OK, I know that cheating the post office out of money is illegal, so don’t do it! I’m not advocating it and please don’t get my thread closed! [/sup] When I mail a letter, I usually put some clear tape over the stamp just to make sure it doesn’t fall off if I didn’t lick it enough. Just a habit that my mom taught me. But I was looking at some of the mail I received at work from abroad and I’ve started saving the pretty foreign stamps. However, inspecting them closely, I notice that some (not all or most) don’t have any indications of the postmark stamp that gets stamped on a letter. Some other ones just have the slightest ink that can’t be seen unless you really scrutinize. So, basically, how can the post office be sure I won’t tape one of these used stamps to my letter and get free postage? It occurred to me that there might be some special ink in the stamp (postmark stamp) that the traces of which can be detected with a machine. Never heard of this thing before, though. Ideas? ---
**Author:** ![drachillix](https://avatars.discourse-cdn.com/v4/letter/d/48db29/32.png) [@drachillix](https://boards.straightdope.com/u/drachillix)\ **Post date:** [December 20, 2002, 11:47pm UTC](https://boards.straightdope.com/t/how-does-the-post-office-know-if-a-stamp-has-been-used/144136/2 "2002-12-20T23:47:50Z")
> [@](#): > > \*Originally posted by Jayrot \* > So, basically, how can the post office be sure I won’t tape one of these used stamps to my letter and get free postage? It occurred to me that there might be some special ink in the stamp (postmark stamp) that the traces of which can be detected with a machine. Never heard of this thing before, though. > Ideas? Well I know a way to defeat postparking which I will not advertise here but generally the postmark stamps here in the US put ink marks on the stamp. Clear tape will most likely damage the stamp if you try to remove it. IIRC collectors usually want uncirculated stamps in usable condition. You might be able to get away with scrubbing postmarks off of the clear tape but I wouldn’t want to make a habit of it. I have a friend who works in a postal encoding center, they are pretty good at backtracing and crossreferencing people if they need to. ---
**Author:** ![Muad\_Dib](https://avatars.discourse-cdn.com/v4/letter/m/35a633/32.png) [@Muad\_Dib](https://boards.straightdope.com/u/Muad_Dib)\ **Post date:** [December 20, 2002, 11:50pm UTC](https://boards.straightdope.com/t/how-does-the-post-office-know-if-a-stamp-has-been-used/144136/3 "2002-12-20T23:50:22Z")
Look at most letters, there will be an ink stamp that covers the paper stamp and extends onto the envelope. ---
**Author:** ![Jayrot](https://avatars.discourse-cdn.com/v4/letter/j/3ec8ea/32.png) [@Jayrot](https://boards.straightdope.com/u/Jayrot)\ **Post date:** [December 20, 2002, 11:59pm UTC](https://boards.straightdope.com/t/how-does-the-post-office-know-if-a-stamp-has-been-used/144136/4 "2002-12-20T23:59:41Z")
> [@](#): > > Look at most letters, there will be an ink stamp that covers the paper stamp and extends onto the envelope. Yes. I understand that. I’m talking about the cases (more often than you imagine) where the ink doesn’t make it to the stamp. > [@](#): > > Clear tape will most likely damage the stamp if you try to remove it. I’m not talking about the stamps that I send. I’m talking about stamps that are sent to me. I only mentioned the tape because most used stamps lose their stickiness. ---
**Author:** ![I\_am\_Sparticus](https://avatars.discourse-cdn.com/v4/letter/i/c68b51/32.png) [@I\_am\_Sparticus](https://boards.straightdope.com/u/I_am_Sparticus)\ **Post date:** [December 21, 2002, 1:30am UTC](https://boards.straightdope.com/t/how-does-the-post-office-know-if-a-stamp-has-been-used/144136/5 "2002-12-21T01:30:25Z")
I believe that stamp collectors, philatilists, wish to have stamps that are postmarked on their first day of issue, or second best, in uncirculated condition, but IANAP ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [December 21, 2002, 1:40am UTC](https://boards.straightdope.com/t/how-does-the-post-office-know-if-a-stamp-has-been-used/144136/6 "2002-12-21T01:40:53Z")
I think putting clear tape (or any tape) over a stamp is illegal too. Actually I had a letter returned to me because I did this. ---
**Author:** ![Ferret\_Herder](https://avatars.discourse-cdn.com/v4/letter/f/e47774/32.png) [@Ferret\_Herder](https://boards.straightdope.com/u/Ferret_Herder)\ **Post date:** [December 21, 2002, 1:52am UTC](https://boards.straightdope.com/t/how-does-the-post-office-know-if-a-stamp-has-been-used/144136/7 "2002-12-21T01:52:12Z")
My husband works for the USPS but as a letter carrier; he does not deal with the cancelling of envelopes. His statement is that supposedly all taped-on stamps should result in the letter being returned to sender, and he was surprised that you’ve gotten a significant number of letters through with taped stamps. ---
**Author:** ![Jayrot](https://avatars.discourse-cdn.com/v4/letter/j/3ec8ea/32.png) [@Jayrot](https://boards.straightdope.com/u/Jayrot)\ **Post date:** [December 21, 2002, 1:54am UTC](https://boards.straightdope.com/t/how-does-the-post-office-know-if-a-stamp-has-been-used/144136/8 "2002-12-21T01:54:37Z")
Really, I didn’t know that. I’ve been doing that pretty much my whole life. I won’t do it anymore though. Thanks **k2dave** and **Ferret Herder** , that pretty much answers my question. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [December 21, 2002, 3:05pm UTC](https://boards.straightdope.com/t/how-does-the-post-office-know-if-a-stamp-has-been-used/144136/9 "2002-12-21T15:05:42Z")
Kust to add to the taped on stamp. If I think the glue is not up to snuff I will sometimes add some tape around the very edges of the stamp and never had a problem. My wife was the one who taped over the stamp and that was the letter that was returned. ---
**Author:** ![Annie-Xmas](https://avatars.discourse-cdn.com/v4/letter/a/ecc23a/32.png) [@Annie-Xmas](https://boards.straightdope.com/u/Annie-Xmas)\ **Post date:** [December 21, 2002, 3:16pm UTC](https://boards.straightdope.com/t/how-does-the-post-office-know-if-a-stamp-has-been-used/144136/10 "2002-12-21T15:16:55Z")
I asked a friend, who happens to be the head of a local post office, what’s to stop people from using stamps that somehow don’t get cancelled. She replied “Nothing.” Not that I’m advocating doing it. ---
**Author:** ![Jake](https://avatars.discourse-cdn.com/v4/letter/j/f475e1/32.png) [@Jake](https://boards.straightdope.com/u/Jake)\ **Post date:** [December 21, 2002, 4:46pm UTC](https://boards.straightdope.com/t/how-does-the-post-office-know-if-a-stamp-has-been-used/144136/11 "2002-12-21T16:46:12Z")
I thought all stamps these days were pre-glued and required no licking? This would of course eliminate the need for tape over the stamp as the self-sticking stamps stick very well. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [December 21, 2002, 10:29pm UTC](https://boards.straightdope.com/t/how-does-the-post-office-know-if-a-stamp-has-been-used/144136/12 "2002-12-21T22:29:05Z")
> [@](#): > > \*Originally posted by Jake \* > I thought all stamps these days were pre-glued and required no licking? This would of course eliminate the need for tape over the stamp as the self-sticking stamps stick very well. No, not all of them. But most of the first class stamps are. Many of the other denominations are not. I can’t believe no one has included a link to this Cecil Classic (not quite on the OP, but close): [Can I reuse the uncanceled stamps from my junk mail?](http://www.straightdope.com/classics/a5_026.html)