Unlike the Spanish milled dollar the U.S. dollar is based upon a decimal system of values. In addition to the dollar the coinage act officially established monetary units of mill or one-thousandth of a dollar (symbol ₥), cent or one-hundredth of a dollar (symbol ¢), dime or one-tenth of a dollar, and eagle or ten dollars, with prescribed weights and composition of gold, silver, or copper for each. It was proposed in the mid-1800s that one hundred dollars be known as a union, but no union coins were ever struck and only patterns for the $50 half union exist. However, only cents are in everyday use as divisions of the dollar; "dime" is used solely as the name of the coin with the value of 10¢, while "eagle" and "mill" are largely unknown to the general public, though mills are sometimes used in matters of tax levies, and gasoline prices are usually in the form of $X.XX9 per gallon, e.g., $3.599, sometimes written as $3.599⁄10. When currently issued in circulating form, denominations equal to or less than a dollar are emitted as U.S. coins while denominations equal to or greater than a dollar are emitted as Federal Reserve notes (with the exception of gold, silver and platinum coins valued up to $100 as legal tender, but worth far more as bullion). Both one-dollar coins and notes are produced today, although the note form is significantly more common. In the past, "paper money" was occasionally issued in denominations less than a dollar (fractional currency) and gold coins were issued for circulation up to the value of $20 (known as the "double eagle", discontinued in the 1930s). The term eagle was used in the Coinage Act of 1792 for the denomination of ten dollars, and subsequently was used in naming gold coins. Paper currency less than one dollar in denomination, known as "fractional currency", was also sometimes pejoratively referred to as "shinplasters". In 1854, James Guthrie, then Secretary of the Treasury, proposed creating $100, $50 and $25 gold coins, which were referred to as a "Union", "Half Union", and "Quarter Union", thus implying a denomination of 1 Union = $100.

Answer this question, if possible (if impossible, reply "unanswerable"): What is the Union based upon?
unanswerable