How Do You Write Twelve Thousands Of An Inch Using Decimals?

Nice pic. Very clear to understand. But I think comma shouldn't be there in the mathematical notation. Maybe American notation.

As nearly as I can tell, the comma is used in the US to mark groups of three digits to the left of the decimal point. In India, numbers seem to be so separated after three digits, then every two digits thereafter. At least some European countries interchange the comma and decimal from what is shown. At least one international publication separates groups of three digits with a thin space. As you say, these separators have no math value; they just help readability.