Every credit card contains a magnetic stripe on
its back. It is made from magnetic iron particles
placed in the plastic film at the back of the card. These particles are formed in such a way
that it behaves like a bar magnet. Since these particles are magnetic in nature
and easily magnetised in South or north direction giving a specific code to the
card. A card reader is used to read the magstrips for validity.

Why your card is not accepted

Sometimes when you shop your card is not accepted. The reason might be
that your magstripe is dirty, scratched, or demagnetized by exposure to any
magnet or a super store's demagnetizer used in electronic surveillance.

Stripe Information

On a magstripe three tracks are made by using the magnetic material. Each of
these tracks has only a width of 1/10 of an inch. The 7811 standard of IEC/ISO
specifies the following specifications for a magstripe.

• Track 1 must have 210 bits in an inch to hold 79 are 6-bit and a read only
  bit for parity.

• Track 2 must have 75 bits in an inch to hold 40 parity bits which are 4-bit
  or higher.

• Track 3 must have 210 bits in an inch to hold 107 parity bits which are
  4-bit or higher.

The first two tracks are typically used as the third track is not standardized
among banks. The third track contains information like encryption pin, currancy
and country codes and credit limit.

Track 1

Two formats are used to store information in track 1. Format A is used for
the proprietary usage of credit card issuing body and second format B contains
the following elements listed in a sequence;

• In the beginning one character is reserved for starting sentinel
• A character is reserved for format type i.e. format code = '"B"'
• 19 characters can be used for primary account number
• A character is used for a separator.
• Three characters are used for country code
• 2 upto 26- twenty six characters for names.
• One character is used for a separator
• After this information 4 characters can be used for expiry date.
• To record Discretionary data there are enough free characters for the complete
  record

• A character is for ending sentinel
• A character is reserved for LRC or longitudinal redundancy check

Track 2

Banking industry has developed the format of the second track. It contains the following information;

• A character is reserved for starting sentinel
• 19 character are used for primary account number
• A character for a separator
• Three characters for country code
• Four characters for expiry date.
• To record Discretionary data there are enough free characters for the complete
  record

• A character is reserved for LRC or longitudinal check for redundancy