Definitions
from The American Heritage® Dictionary of the English Language, 4th Edition
 adj. Of, relating to, or based on the number 12: the duodecimal number system.
 adj. Of or relating to twelfths.
 n. A twelfth.
from Wiktionary, Creative Commons Attribution/ShareAlike License
 adj. Of a number, expressed in base 12.
 n. A number system that uses 12 as its base.
from the GNU version of the Collaborative International Dictionary of English
 adj. Proceeding in computation by twelves; expressed in the scale of twelves.
 n. A twelfth part.
 n. A system of numbers, whose denominations rise in a scale of twelves, as of feet and inches. The system is used chiefly by artificers in computing the superficial and solid contents of their work.
from The Century Dictionary and Cyclopedia
 Reckoning by twelves and powers of twelve: as, duodecimal multiplication.
 n. One of a system of numerals the base of which is twelve.
 n. plural An arithmetical rule for ascertaining the number of square feet, twelfths of feet, and square inches in a rectangular area or surface whose sides are given in feet and inches and twelfths of inches.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 n. one part in twelve equal parts
 adj. based on twelve
Etymologies
from The American Heritage® Dictionary of the English Language, 4th Edition
Examples

Up and after ordering some things towards my wife's going into the country, to the office, where I spent the morning upon my measuring rules very pleasantly till noon, and then comes Creed and he and I talked about mathematiques, and he tells me of a way found out by Mr. Jonas Moore which he calls duodecimal arithmetique, which is properly applied to measuring, where all is ordered by inches, which are 12 in

Up and after ordering some things towards my wife's going into the country, to the office, where I spent the morning upon my measuring rules very pleasantly till noon, and then comes Creed and he and I talked about mathematiques, and he tells me of a way found out by Mr. Jonas Moore which he calls duodecimal arithmetique, which is properly applied to measuring, where all is ordered by inches, which are 12 in a foot, which I have a mind to learn.

It later changed its name to the Dozenal Society of America, since the word “duodecimal” was deemed to be overly reminiscent of the system they were aiming to replace.

In arithmetic, and in other disciplines as well, truths remain the same even if notations are changed, and it does not matter whether a decimal or a duodecimal number system is used (Leibniz 1670, 128).

Utopian of all things, a duodecimal system of counting.

Our numbering system was a duodecimal one, and so it fit right in.

Two possible number systems that have, for purely theoretical reasons, attracted much attention, are the octonary and the duodecimal systems.

Aside from our common decimal scale, there exist in the English language other methods of counting, some of them formal enough to be dignified by the term _system_  as the sexagesimal method of measuring time and angular magnitude; and the duodecimal system of reckoning, so extensively used in buying and selling.

The duodecimal is not a natural scale in the same sense as are the quinary, the decimal, and the vigesimal; but it is a system which is called into being long after the complete development of one of the natural systems, solely because of the simple and familiar fractions into which its base is divided.

From a commercial standpoint this advantage is very great; so great that many have seriously advocated the entire abolition of the decimal scale, and the substitution of the duodecimal in its stead.
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