\section{Arm: The generic component}
\label{s:arm}
\index{Optics!Point in space (Arm, Optical bench, Coordinate system)}
\mcdoccomp{optics/Arm.parms}

The component \textbf{Arm} is empty; is resembles an optical bench
and has no effect on the x-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 \MCX\ meta-language.
The use of \textbf{Arm} components in beamline 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
\textbf{Progress\_bar} (sec.~\ref{s:progress-bar}) component in order to display
simulation progress on the fly, and possibly save intermediate results.
