Living in America means you are inundated by all kinds of numbers. You are ultimately a number in the system (think Big Brother) and this number is called your social security number (the ironies and paradoxes in that moniker are food for another post). Then, since you need to contribute to the economy and the tilted scales of demand vs supply, you get one or more bank accounts and one or a pack of credit cards. More numbers. Then there are debit card PINs, credit card security numbers, toll-free customer service[sic] numbers. Admittedly, there are established and adequate mechanisms to deal with lists of phone number, important dates and such.
Which brings me to some questions that inevitably popped up as I got used to this system: what does the format of a Social Security Number (SSN) mean (if anything at all)? and the format of a credit card number? and how do they validate and cross-check? A recent technical article contained a useful phrase "the LUHN formula". This sent me googling, and I found a page that offered an explanation. And another one provided a description with a programmatic bent (and made even more sense). And then there was the ever-reliable Wikipedia page, which also links to the number patterns of different credit card companies.
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