Code 128 barcode is a high-density linear symbology that encodes text, number, numerous functions and the entire 128 ASCII character set.  This is a very effective symbology for use where a relatively large amount of data must be encoded in a relatively small amount of space.  Its specific structure allows for numeric data to be essentially, encoded at double-density.

The parts of the 128 barcode are the "quiet zone", one of three start codes, the data itself, a check character, a stop character and a trailing "quiet zone".

Quiet Zone - the clear area, free from marks, before and after the bars and spaces.  This allows the scanner to establish base values.

Start Code - is one of three codes that signal the start of a Code 128 barcode and specifically defines which character set will be be used.

Data - is encoded in strips of bars and spaces. It is done using a binary system to represent the encoding, using a "1" for a single-width bar and a "0" for a space.

Check Character - is a digit that verifies that the data just read, was correct  The check/sum digit is a calculation based on the sum of the values of each of the digits in the message that is being encoded.

Stop Code - is the bar-and-space sequence that signals the end of the barcode.