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.
As to length, the longest extant bony fish on earth is the king of herrings or oarfish (Regalecus glesne). Slender and compressed, this fish averages over 6 m (20 ft) long at maturity. A specimen caught in 1885 of 7.6 m (25 ft) in length weighed 275 kg (606 lb).