Ever wonder what those barcodes at the bottom of almost every envelope you get mean?

This information is as found in USPS Publication 25 found at usps.gov

The barcodes are used by mail sorting machines to speed the process of mail delivery. The machine is able to read the barcodes and sort mail into the proper bag, carton, or box for shipment or delivery.

The bars are binary in nature. They are either long or short. Five bars are required to represent one digit. For each digit, there will be two long bars and three short bars. The placement and order of the bars define which number it represents.

There are 62 bars on most envelopes. This represents 12 numbers. The first and last bars are always long bars and do not represent a number, but simply indicate the beginning and end of a barcode.

The first nine digits are the ZIP+4 code.

The next two digits (#10 and #11) are the USPS delivery code, which can be found at usps.gov for virtually any address in America.

The last digit (#12) is a control digit. The verifies that the previous 11 digits were correct. All 12 digits must add together to be a multiple of 10. The last digit is meant to force this sum.

Each digit is represented by five bars: two long bars and three short bars. The first bar signifies 7, the second bar 4, the third bar 2, the fourth bar 1, and the last bar is 0.

The digit 5, for example, is created by placing a long bar second and fourth, meaning 4 and 1.

Therefore, add where the long bars appear, and you have the corresponding digit (except for zero, which is made by placing a long bar at 7 and 4.)

The following table shows the digits as they appear in barcodes.

1			6
2			7
3			8
4			9
5			0