“Visa provides an infrastructure … in which multiple competing financial institutions can cooperate, just enough, to provide a service that none could have realistically provided alone,” Stearns wrote. “In short, Visa makes money move.”

Visa wasn’t the only such organization. A different consortium of financial institutions banded together into Master Charge, which became Mastercard. Then those systems learned to work together (perhaps too well, according to retailers who have long-running antitrust litigation against the companies).

But there was one other key step in creating the modern point-of-sale system. Flip your card over now. Take a look at the magnetic stripe toward the top. It’s what made your card machine-readable. The system was developed by IBM in the late 1960s, and according to one of its architects, “The original information standards—the way the data is physically laid out on the mag stripe—has survived every migration of transaction media, from mag-stripe cards to smart cards, from smart cards to smartphones.”

The stripe itself is not unlike the tape in the cassettes you put in a boom box. But instead of encoding music in a form that can be played back from your shoulder, that little strip of iron oxide contains your account number. Swiping it through a reader plays it back. That’s why Square’s original card reader was designed attached to the headphone jack: The whole device merely sent the signal from the audio read head to the mic input, and then the app could take it from there. (Some beautiful nerds took advantage of this capability and converted the Square reader into a kind of instrument.)

“The payment card is merely an access device, a means for identifying the cardholder to the vast electronic financial network that lies behind it,” Stearns wrote in an essay in Paid: Tales of Dongles, Checks, and Other Money Stuff. Your card is a fob for walking into the vast digital storehouse where the “alphanumeric data” formerly known as money is kept.

From the very beginning, American credit cards have been relatively insecure. If Square can build a dirt-cheap way to read your credit card, so can fraudsters who built devices called “skimmers” which can steal the data off cards, reencode it onto a new one, and, voila, someone is swiping their way around with your account. You might think your signature is a security measure, but it’s basically theater. The only real security in the system is on the network level, where banks process transaction data to look for “suspicious activity.”

Chip-card technology—known as EMV in the industry—is more secure. The data can’t be easily skimmed from the chip, as it stores important information in an encrypted format. For these reasons, it’s been standard in Europe for more than two decades. But not in the United States.

Some of that was timing. One, if you installed a brand-new system in the 1970s, you probably didn’t want to buy all new hardware in the 1980s. Two, swiping is super fast and super easy. “Swiping is a really good experience,” said Jesse Dorogusker, the head of hardware at Square. “It sets a really high bar for convenience and speed.” Even if Square can spend the development resources to get its chip-card processing down to three seconds, other systems might take much longer—try counting, you might get to 10 or even 12 sometimes. “It makes for an inconsistent ecosystem,” Dorogusker said. Three, IBM, the developer of the mag-stripe card, was in the database business, so promoting more back-end data processing seemed like a good idea to them.