\section{Arm: The generic component}
\label{explain:arm}
\component{Arm}{System}{(none)}{(none)}{}
\index{Optics!Point in space (Arm, Optical bench)}

The component \textbf{Arm} is empty; is resembles an optical bench
and has no effect on the neutron ray.
The purpose of this component is only to provide a standard
means of defining a local coordinate system within the instrument definition.
Other components may then be
positioned relative to the \textbf{Arm} component
using the \MCS meta-language.
The use of \textrm{Arm} components in the instrument definitions
is not required but is recommended for clarity.
\textbf{Arm} has no input parameters.

The first Arm instance in an instrument definition may be changed into a \verb+Progress_bar+ component in order to display on the fly the simulation progress, and possibly save intermediate results.
