\begin{definition}[Definition:Real Interval/Notation]
An arbitrary (real) interval is frequently denoted $\mathbb I$.
Sources which use the $\textbf {boldface}$ font for the number sets $\N, \Z, \Q, \R, \C$ tend also to use $\mathbf I$ for this entity.
Some sources merely use the ordinary $\textit {italic}$ font $I$.
Some sources prefer to use $J$.
\end{definition}