Postal codes have nine digits because the United States Postal Service determined this was the best way to ensure precise locations for mail delivery. ZIP codes are numerical codes used to designate zones for mail delivery.
The traditional postal code for the United States has five digits.The first three digits of the code are used to determine which region of the country the mail is being sent to. ZIP codes begin with 0 and go through 9 from east to west. Senders can use an additional four digits, but these are not necessary for mail to be delivered to the recipient. These last four digits are used to reference very specific locations, such as city blocks.