\begin{definition}[Definition:Duodecimal Notation]
'''Duodecimal notation''' is the technique of expressing numbers in base $12$.
Every number $x \in \R$ is expressed in the form:
:$\ds x = \sum_{j \mathop \in \Z} r_j 12^j$
where:
:$\forall j \in \Z: r_j \in \set {0, 1, \ldots, 10, 11}$
In order to be able to represent numbers in such a format conveniently and readably, it is necessary to render the digits $10$ and $11$ using single characters.
There is no universal convention for this, but a common approach is to use the following:
{{begin-eqn}}
{{eqn | l = 10
      | o = :
      | r = \mathrm T
}}
{{eqn | l = 11
      | o = :
      | r = \mathrm E
}}
{{end-eqn}}
that is, the initial letters of '''ten''' and  '''eleven'''.
\end{definition}