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.
Length is the distance from one end to the other end of an object. There are two systems for the measurement of length. One is the metric system and the other is customary system. The units of metric system are centimeter, meter, kilometer, millimeter, etc.