A few U.S. dollar bills bear strange marks — small stamps, pentagrams or a small pair of initials not found on the official symbol. People call them chop marks, and they bear witness to a global journey through foreign markets and money exchanges.

What Are Chop Marks?
Chop marks are small, clipped symbols stamped on U.S. bills by money changers or currency handlers. They are an indication that a bill has been scrutinised and found to be authentic. Widespread in Southeast Asia, Africa and Latin America, these patterns of markings help ease distrust in economies where fake bills are more common.
From China to the Ottoman Empire The word “chop” has its roots in China, a world leader in production of stamped silver coin, and thousands of merchants competed with one another to produce the best-stamped coin as unequivocal evidence of its weight and authenticity. People have incorporated paper money into the practice, especially the American dollar, which is so widely used outside the United States.

Source: Flickr
Why Do The Money Changers Chop Marks?
Verifiers stamp chop marks on a bill after it passes the ordinary security checks. Someone marks the chit with a specific sign to indicate provenance. This process:
Verifies to future possessor the legitimacy of the bill.
Monitors its movement along local financial chains
Builds trust where there is no strong central verification
What Do These Marks Appear As?
Appearance of Chop marks:

Arrows, stars or birds and other symbols
Stylized letters or initials
Special symbols such as one of a bow and arrow, as on a $20 bill recently

Source: Flickr
They use ink — blue, black, or red — to prevent obscuring the official security elements.
Are Chop Marks Legal?
Federal statutes in the United States outlaw the mutilation of currency to the point of affecting its value as legal tender, but chop marks do not rise to that level. These tiny stamps have no impact on the functioning or value of a bill. But machines and some banks may refuse heavily defaced bills because of appearance.

A Quiet Story in Your Wallet
A marked bill might have run through markets, banks or booths so some distant land. They’re all stories about verification and movement and trust across borders. When you receive one, you’re holding a slice of that global expedition.