The check digit is used to confirm the initial digits of the card number. The benefits of using a checksum is that it prevents casual attempts to invent credit card numbers, as only one in ten will be valid.
Length (adj). the longest, or longer, dimension of any object, in distinction from breadth or width; extent of anything from end to end; the longest line which can be drawn through a body, parallel to its sides; as, the length of a church, or of a ship; the length of a rope or line.
http://www.thelongestdomainnameintheworldandthensomeandthensomemoreandmore.com/ claims to be the world's longest domain name. But is it? It turns out that this is a matter of definition. According to the domain registrars, the longest legal domain name is 63 characters starting with a letter or number.