Elements of a Bar Code

Almost all bar codes contain the following elements:

Start and Stop Symbologies

At the beginning and end of some bar code symbols, there are "start" and "stop" characters. These characters identify the symbology and also enable the scanner to read the symbol bi-directionally, decoding the data in the correct order.   Bar codes also often include a check digit at the end that is determined according to an algorithm based upon the preceding characters.

Quiet Zones

In order for the scanner to recognize the bar code, there must be an 1/4" wide area next to the start and stop characters that contains no markings.  If the space is too short, the bar code symbol will not be read by the scanning device.

Interpretation Line

This is a line of human-readable characters that is located usually beneath the bar code.  An example is shown in the Code 128.

Bar/Space Patterns

These are the wide and narrow black bars and whites spaces contained in the bar code.

Inter-Character Gap

In some bar codes, like Code 39 (same as Code 3 of 9), each character is printed independently of other characters and is not part of the encoded character and is thus separated by what's called an inter-character gap.

Code Density

Code density refers to the number of data or message characters which can be represented per unit length of space.   Four variables affect code density:  type of code, ratio of wide to narrow elements and the X dimension

Ratio of Wide to Narrow Elements

Ratio is important in deciding on the type of scanning device to read the code.  Changing the code density of a particular bar code is accomplished by varying the ratio.

X Dimension

The width of the narrowest bar or space is referred to as the X dimension, usually given in mils (thousandths of an inch). The X dimension dictates the width of all other bars and spaces, and ultimately the length of the bar code. The greater the X dimension, the more easily a bar code will scan. The smaller the width of the bar, the shorter the length of the symbol, the closer the tolerances are and the more difficult it is to print.  The larger the width of the elements, the more space it takes to print the bar code; therefore, the lower the bar code density.  The thinner the bar and spaces, the less space is required and the higher the bar code density.  Lower density bar codes are more reliably printed and more consistently read than higher density bar codes.