TOUR Piercing Tools Jewelry Index 2015  Unimax
Measurements and Gauge    2015  Unimax Commonly used measurements and apporximations.
History was Made - UNIMAX was there for modern piercing.
The gauge sizes of American Jewelry are the result of availability of product rather than a pre thought-out scheme of what would be suitable for piercing. Piercers today use even numbered gauge sizes (14-12-10-8, etc.) because odd numbered gauge sizes (13-11-9, etc.) were not normally used in wire applications so the material was not generally available for purchase.
For jewelry manufacturing what was available became the standard rather than what made sense for piercing jewelry. It was only when Asia became an affordable choice did jewelry sizes begin selling in millimeter sizes, an orderly progressive range of sizes.
AWG = B&S,   One "mil" = 1/1000 of an inch
Historically the US has used the American Wire Gauge (AWG) system, also known as Brown & Sharpe (B&S) Gauge system which is based on the division of an inch into 1000 units (thousands of an inch). 
The division of the inch (a standard unit of measurement in US) is required because very small sizes need to be measured and calculated with accuracy and a base of 10 is much easier to calculate with, rather than a base of 12.  A shorthand unit is used to refer to "one one thousand of an inch" (a mouthful) and is called a "mil." (Not to be confused with mm for millimeter, or ml milliliter.) Use of this term is similar to other very familiar terms; like foot, acre and pound.  For example, it is easier to talk, write and think about 2 "miles" than to calculate and use how many feet this would be once you approach larger numbers. Consider the difficulty of measuring mileage per gallon of gas in feet or inches rather than in miles per gallon.  Because calculations in electronics deal with numbers that can be 18 digits long and more, it is much easier to use smaller representational units and limit the number to three digits followed by the "shorthand" unit unless greater accuracy is needed.
In AWG the bigger (larger) the number, 
the smaller the size.
In the AWG system the larger the number, the smaller the size.  For example, a # 2 wire (2 Gauge) is nearly as large as a pencil. A # 44 wire is about the thickness of a human hair.  A 22 gauge wire can be used for electrical circuits; 16 and 18 gauges are common lamp cord sizes; and 14 gauge is the smallest size permissible for permanent wire installation.
Wires larger than 0-gauge are labeled 00 (2/0-twice the cm of 0), 000 (3/0-three times the cm of 0) and 0000 (4/0-four times the cm of 0).  Larger than this they are classified by their actual cross-sectional area (cm = "circular mil").
Looking at your gauge wheel the gauge size or number appears on one side, e.g. 14G. On the other size is the corresponding size in thousands of an inch, .064", and is 64 thousands of an inch or in shorthand speak: 64 mils.
The European system is Metric, based on dividing a unit of length into divisions of 10.  It is clear to see this would be an advantage for easy calculation.  Our efforts at Unimax have been focused on providing the leading edge of thinking and promotion of the science of piercing.