Hidden square on center on polymer bills

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Does anyone no there reason why ther is a approx. 1.5cm square in the center of the bills? You cant really see it unless you adjust the bill to the lighting.  Once you find it you will be able to spot it right away.  I did not check the $5 and $10 bills but it is on all other bills.  I sure would like to know whay it is there. 

I have since had the chance to look at the $5 and $10 and I am not able to see it on those bills.  As I said before the square is on the other bills.  It is on the back of the bills and to help pinpoint it a bit better, if you look at the $20 it is on the monument to the right about half way up.  play with the light by tilting the bill and you will see it.  Some lighting is better than the other but direct sun light seems to be the hardest light to find it in.  Am I going crazy or does anyone else see it?  Ernest

if you do a site seach it has been mentioned before...I'm not sure if any answers came up but it does exist

Thanks for the tip.  I did do a search but there was no answer as to the square just more people who have seen it too.  Take care.  Ernest

I can't see the square on the $10. I believe I can make out a slight outline of it on the $5 but it might just be my eyes playing tricks on my.

As for he $20 and up, all we know is that it's there; its purpose remains unknown to the public at this time. Many believe it to be an undocumented security feature.