To really make the cashless society convenient and "identifiable" there is nothing like a bar code. Just about everything you can buy has a "bar code" on it. You know, those liney things they scan at the supermarket... Bar codes come in several formats and flavors, but when it comes to retail operations and consumer applications, the most common kinds are the UPC/EAN-13 format...

In most bar codes, numbers are coded as two bars. The length of the bars up and down does not matter to the computer. What does matter is the thickness of the lines and the thickness of the gaps between them. You'll also notice two long skinny lines on the left, in the middle, and on the right hand side. These are referred to as start and stop scan codes. On the above example above, you can see the start codes at the left, in the middle and the right. You can also see them even more clearly on the example below.

The code patterns used on the left 5 numbers are different from the ones on the right 5 numbers.
 

The UPC/EAN-13 Specification or "the common barcode" 

Each line used is either fat, medium, or skinny. Gaps appear to be likewise. If you look at the example to the left you'll see the two start code lines with a circle drawn around the bottom ends. You can also see the middle and stop scan codes with the same circles. You can see that a 0 is a medium and skinny line, a 1 is two medium lines, etc., etc.

Notice the 6 on the right side. Two skinny lines. Can you see any difference between the 6 and the start and stop scan codes? Any difference you see between them is due to the limitations of the graphics program I used to scan and enhance the images.

Why would two skinny lines represent a 6? Maybe it's because in pure computer code (binary), a 6 is represented as 0110...