Martin Craft, age 11, of Greenville, Miss., for his question:

WHEN DID WE GET ZIP CODES?

A ZIP code in the address on an envelope helps to speed the sorting and delivery of mail. The name stands for Zoning Improvement Plan.

The Post Office Department, which is now the United States Postal Service, introduced the ZIP code system in 1963. By that time, the volume of mail in the nation had increased by almost 900 percent since 1900. Also, the mail had changed chiefly from personal messages to about 80 percent business correspondence.

Five numbers were added to the address after a state's name. The first number designates one of 10 geographical areas, the second two numbers indicate a metropolitan area or sectional center, and the last two numbers represent a small town or delivery unit from which the mail will be delivered.

Today many ZIP codes have nine numbers. The first five have been joined by an additional four that pinpoint the neighborhood, street and house.