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\bf is an outdated font switch, not a macro with an argument, i.e. itr works to the end of the group. You would need to use {\bf NTn} instead, but should not use the font commands any longer. Use \textbf{..} or {\bfseries ..} instead. – Martin Scharrer♦May 16 '12 at 7:45
You should not use \bf. Instead use {\bfseries ....}.
\documentclass{article}
\usepackage{amsmath}
\begin{document}
{\bfseries NTn} = Number of lines
$\boldmath{NTn} + \mathbf{NTn} + \boldsymbol{\pi} + \pmb{\pi} $ = Number of lines.
\verb|\boldsymbol| and \verb|\pmb| are provided by \verb|amsmath| package. Note that \verb|\mathbf| has no effect on symbols like $+$ or $-$ or $\pi$ etc.
\end{document}
\textbf{Some text in bold}. – Ali Mehrizi May 16 '12 at 5:16\textitor\it,\bfseriesor\bf, etc. and Will two-letter font style commands (\bf,\it, …) ever be resurrected in LaTeX? regarding the use of font switches. – Werner May 16 '12 at 5:35\bfis an outdated font switch, not a macro with an argument, i.e. itr works to the end of the group. You would need to use{\bf NTn}instead, but should not use the font commands any longer. Use\textbf{..}or{\bfseries ..}instead. – Martin Scharrer♦ May 16 '12 at 7:45