Your credit card is more than just the 16 digits you use for transactions. Each symbol and number on your card serves a purpose, whether for fraud protection or simplifying transactions.

Metal cards were first mythologized by the invitation-only American Express Centurion Card, a.k.a. the Black Card, which reportedly boasts a $5,000 annual fee and a $10,000 initiation fee.

After its plastic introduction in 1999, the Centurion Card—or the Black Card, in popular parlance—became a status symbol known far outside its rarefied clientele, largely thanks to countless ...