Multiple section styles in same document

sectioning

I have a multipart book. I want to create different styles for different chapters. Based on this, I worked so far:

\documentclass[10pt]{book}
\usepackage{titlesec}

%original definition Style A
\titleformat{\chapter}[display]
  {\normalfont\bfseries\filcenter}{\LARGE\thechapter}{1ex}
  {\titlerule[2pt]\vspace{2ex}}[\vspace{1ex}{\titlerule[2pt]}]
\titleformat{name=\chapter,numberless}[display]
  {\normalfont\LARGE\bfseries\filcenter}{}{1ex}
  {\vspace{2ex}}[\vspace{1ex}]
%end of original definition

\begin{document}

\part{First Part}
THis
\chapter{Chapter one}
\chapter{Chapter two}

%another definition Style B
\titleformat{\chapter}[display]
{\normalfont\huge\bfseries}{\filcenter\underline{\MakeUppercase{{\chaptertitlename}}\ \thechapter}}{20pt}{\Huge}
%end of other definition

\part{Second Part}

\chapter{Chapter three}
\chapter{Chapter four}


\titleformat{\chapter}[display]
  {\normalfont\bfseries\filcenter}{\LARGE\thechapter}{1ex}
  {\titlerule[2pt]\vspace{2ex}}[\vspace{1ex}{\titlerule[2pt]}]
  
\chapter{Chapter five}

\end{document}

Problem is: Every time I have to set definition for new styles. My chapters will switch within frequently: Style A, Style B, Back to Style A, Style B, Even Style C (not implemented in code), Style A, Style B etc.

You see in code definition Style A is working on two chapters in Part 1, then definition Style B is applied on next two chapters, again next chapters uses definition Style A.

Is there any way to make this more organized that I can easily switch between styles for the chapters?

Best Answer

Define two commands \chapterA to apply to chapters style A and a \chapterB for style B.

a

b

\documentclass[10pt]{book}
\usepackage{titlesec}

%%original definition Style A

%\titleformat{\chapter}[display]
%{\normalfont\bfseries\filcenter}{\LARGE\thechapter}{1ex}
%{\titlerule[2pt]\vspace{2ex}}[\vspace{1ex}{\titlerule[2pt]}]

\titleformat{name=\chapter,numberless}[display]
{\normalfont\LARGE\bfseries\filcenter}{}{1ex}
{\vspace{2ex}}[\vspace{1ex}]

%%end of original definition

\newcommand{\chapterA}[1]{% style A <<<<<<<<<<
    \titleformat{\chapter}[display]
    {\normalfont\bfseries\filcenter}{\LARGE\thechapter}{1ex}
    {\titlerule[2pt]\vspace{2ex}}[\vspace{1ex}{\titlerule[2pt]}]
    \chapter{#1}
}

\newcommand{\chapterB}[1]{% style B <<<<<<<<<<
    \titleformat{\chapter}[display]
    {\normalfont\huge\bfseries}{\filcenter\underline{\MakeUppercase{{\chaptertitlename}}\ \thechapter}}{20pt}{\Huge}
    \chapter{#1}
}

\begin{document}
    
    \part{First Part}
    THis
    \chapterA{Chapter one}
    \chapterA{Chapter two}
    
%   %another definition Style B
%   \titleformat{\chapter}[display]
%   {\normalfont\huge\bfseries}{\filcenter\underline{\MakeUppercase{{\chaptertitlename}}\ \thechapter}}{20pt}{\Huge}
%   %end of other definition
    
    \part{Second Part}
    
    \chapterB{Chapter three}
    \chapterB{Chapter four}
    
    
%   \titleformat{\chapter}[display]
%   {\normalfont\bfseries\filcenter}{\LARGE\thechapter}{1ex}
%   {\titlerule[2pt]\vspace{2ex}}[\vspace{1ex}{\titlerule[2pt]}]
    
    \chapterA{Chapter five}
    
\end{document}
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