ID number consisting of

Citizenship applications not only in the number of identity cards, also used in other areas. Basically, birth date code, order and gender codes, and check these things constituted by the address code ID cards. ID cards are usually the last digit, why would it appear the letter X? The reason is the number of identity card is prepared in accordance with national standards, it consists of 18 bits,? Stand for the first 18 check code. ? Check code is a number Prepared by the system -? - a formula calculated, if someone's tail number is 0-9, X will not occur, but if the tail number is 10, so you have to use X instead of, because if done with tail number 10, then the person's identity becomes 19, and 19 numbers do not meet national standards. X is the Roman numeral 10, it is used instead of the number 10, the identity meet the standards!