Magnetic stripe card
The standard ISO 7811 specifies the format in which information is stored on these cards. A magnetic stripe holds about 1024 bits of information, which is arranged in three tracks. The first track holds 76 alphanumeric characters, encoded in 7 bit per character. The two other tracks  hold 37 and 104 numeric characters, encoded with 5 bits per character. The first two tracks can only be read, the third track can also be written to.