In a recent Slate article, writer Christopher Beam states that…

We got the $ from the Spanish. In the late 18th century, merchants in the North American British colonies traded mainly with two currencies: the British pound and the Spanish dollar. When the United States adopted its own currency in 1785, it used Spanish money as its model–a deliberate “screw you” to the British. Scholars have since theorized that the $ sign evolved out of an abbreviation for peso: The plural for pesos was “ps,” which eventually became “ps,” and then simply an “S” with a single stroke denoting the “p.”

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