Q:

How to define an operator with an index argument below? 

I am defining a trace symbol as follows:
\DeclareMathOperator{\Tr}{Tr}

but now I would like to be able to insert a summation variable below.  However \Tr_x places the subscript x on the side:

I want the subscript below, like this:

I know I can achieve this by typing \underset{x}{\Tr}, but that's a long command.  Is there a way to have the subscript placed below by default whenever I use \Tr?  Or another way of defining the operator/command that achieves this easily? 

A:

In order to get behavior like with \lim use the starred version
\DeclareMathOperator*{\Tr}{Tr}

Remember typeset under is only seen in display math, in text style math it is still written on the side as it should or it would disturb line distance. 
\documentclass{article}
\usepackage{amsmath}
\DeclareMathOperator*{\Tr}{Tr}
\begin{document}
$\Tr_x(b)$

\[
\Tr_x(b)
\]

\end{document}

