 The first digit designates one of nine postal regions; the following two digits designate postal distribution centers; the fourth digit is 0 if it belongs to a capital of municipality, 5 if not, or any other digit if it is a designated address; the last three digits sort building blocks and designated addresses.The more important the city, the more rounded is the number formed by the first four digits.Prior to 1976, only Lisbon had used a system, of six zones (Lisboa 1 to Lisboa 6).