# How can I change the space between lines in theorem?

I am using the amsthm package. I need to change the space between the lines of the paragraph in the body of the all theorems only in the theorem environment and not in the other parts of my report..

How can I define the \newtheoremstyle for this purpose? or any other option?

\documentclass[a4paper,fleqn]{report}‎‎
\usepackage{amsthm,amssymb,amsmath} ‎
\begin{document}‎           % Writing math‎
\theoremstyle{plain}
\newtheorem{theorem}{Theorem}‎[chapter]‎
\begin{theorem}‎‎
Balah Blah Blah
Balah Blah Blah
Balah Blah Blah
Balah Blah Blah
Balah Blah Blah
Balah Blah Blah
Balah Blah Blah
Blah Blah   %How to increase between lines?
\end{theorem}
\end{document}

• A minimum working example was added – Shahram Mohammadi Oct 30 '18 at 7:14
• I edited again and tested the example. Now it works – Shahram Mohammadi Oct 30 '18 at 7:23

Modifying the plain style paramenteres, (see this answer by egreg), you could add \onehalfspacing (or \doublespacing) from setspace package.

\documentclass[a4paper,fleqn]{report}
\usepackage{setspace}
\usepackage{amsthm,amssymb,amsmath}

\newtheoremstyle{myplain}
{\topsep}   % ABOVESPACE
{\topsep}   % BELOWSPACE
{\itshape\onehalfspacing}  % BODYFONT
{0pt}       % INDENT (empty value is the same as 0pt)
{5pt plus 1pt minus 1pt} % HEADSPACE
{}
\theoremstyle{myplain}
\newtheorem{theorem}{Theorem}[chapter]

\usepackage{mwe}% only for testing

\begin{document}
\chapter{A Chapter}
\section{A Section}
\blindtext%only for testing
\begin{theorem}
Balah Blah Blah
Balah Blah Blah
Balah Blah Blah
Balah Blah Blah
Balah Blah Blah
Balah Blah Blah
Balah Blah Blah
Balah Blah Blah
Balah Blah Blah
Balah Blah Blah
Balah Blah Blah
Balah Blah Blah
Balah Blah Blah
Balah Blah Blah
Blah Blah
\end{theorem}
\blindtext%only for testing
\end{document}


Edit: I don't know how to set the linespacing exactly in cm, but you can put any value you want in \setstretch{...}.

\documentclass[a4paper,fleqn]{report}
\usepackage{setspace}
\usepackage{amsthm,amssymb,amsmath}

\newtheoremstyle{myplain}
{-\baselineskip\topsep}   % ABOVESPACE
{\topsep}   % BELOWSPACE
{\itshape\setstretch{2}}  % BODYFONT
{0pt}       % INDENT (empty value is the same as 0pt)
{5pt plus 1pt minus 1pt} % HEADSPACE
{}
\theoremstyle{myplain}
\newtheorem{theorem}{Theorem}[chapter]

\usepackage{mwe}% only for testing

\begin{document}
\chapter{A Chapter}
\section{A Section}
\blindtext%only for testing
\begin{theorem}
Balah Blah Blah
Balah Blah Blah
Balah Blah Blah
Balah Blah Blah
Balah Blah Blah
Balah Blah Blah
Balah Blah Blah
Balah Blah Blah
Balah Blah Blah
Balah Blah Blah
Balah Blah Blah
Balah Blah Blah
Balah Blah Blah
Balah Blah Blah
Blah Blah
\end{theorem}
\blindtext%only for testing
\end{document}


• Thanks for the answer but is there a way to define the space exactly by centimeters? – Shahram Mohammadi Oct 30 '18 at 12:24
• @ShahramMohammadi See my edit, I don't know how to set the line spacing exactly in cm, but you can choose any value you want with \setstretch{...}. – CarLaTeX Oct 30 '18 at 14:21