Adding Chapter Subtitle

Question

I would like to include a chapter subtitle line (not included in ToC) immediately below the chapter name, and I cannot get any of the few recommendations I've found to jive with my existing code for the chapter title layout. Any suggestions? Many thanks! (If it helps, I was trying to get my chapter headings to exclude the words "Chapter N" and to be centered.)

\documentclass[11pt]{book}
\usepackage{titlesec}
\pagestyle{plain}

\titleformat{\chapter}[display]
  {\normalfont\bfseries}{}{0pt}{\LARGE\center}
  \titlespacing*{\chapter}{0pt}{-20pt}{40pt}

\begin{document}

\chapter{This is a chapter}{with your idea of how to make this happen}

\end{document}

Answer 1

Try this:

\documentclass[11pt]{book}
\usepackage{titlesec}
\pagestyle{plain}
\titleformat{\chapter}[display]
{\normalfont\bfseries}{}{0pt}{\LARGE\center}
\titlespacing*{\chapter}{0pt}{-20pt}{40pt}
\begin{document}
    \tableofcontents
    
    \chapter[This is a chapter]{This is a chapter\\{\large  with not so long sub title}}
    
\end{document}

Fist page:

Third page (the second is empty):

The content of the '[]' is the short title of the chapter (for TOC and heather/footer).

The content of the external couple of '{}' is the long title that I have dropped (with the double backslash) in two parts: the title and the subtitle (with the smaller chars).

EDIT If You want the subtitle emphasized consider this change:

\chapter[This is a chapter]{This is a chapter\\{\large \emph{ with not so long sub title}}}