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If you go through security at the airport with a paper boarding pass, the TSA will look at your ID and then sign the pass with a pen, but if you use a mobile boarding pass, there is nothing for them to sign (At least as I understand it. Maybe I'm wrong.) Why is the signing necessary in the one case and not the other? Where is the security compensated for in the mobile case? Or what purpose does signing the paper pass serve?

marked as duplicate by Nate Eldredge, Giorgio, CGCampbell, Ali Awan, JonathanReez Jun 28 '17 at 11:16

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    The flaw in your question is that you assume that everything TSA does must have a useful or necessary purpose. There are many things they do that are quite useless. – Zach Lipton Jun 27 '17 at 2:09

According to this article, the TSA officer signs your printed boarding pass so that if questioned, TSA knows who checked you through.

...depending on the airport and the management at the airport, there may also be further information, such as the number of the checkpoint.

The article also states that some TSA officers purposely make the marking illegible just in case they let someone through who needed additional screening, management wouldn't be able to trace it back to that officer. The article essentially states that this process is silly because it can't be done with mobile or digital boarding passes.

...It’s all weird, since they let phone boarding passes through with no marks...

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