# Mixed chapter numbering (e.g. Chapter 1a, Chapter 1b, Chapter 2 ...)

I'm stitching 4 research papers using the book class. The first two papers are closely related, so I would like the chapter labelling to be as following:

• Chapter 1a
• Chapter 1b
• Chapter 2
• Chapter 3

Is there a way to achieve this mixed numbering with some LaTeX variant?

By redefining \thechapter and altering the value of counter chapter, the following example works, though not very elegant.

\documentclass{book}

\begin{document}
\tableofcontents

\renewcommand{\thechapter}{1\alph{chapter}}
\chapter{title}

\chapter{title}

\setcounter{chapter}{1}
\renewcommand{\thechapter}{\arabic{chapter}}
\chapter{title}

\chapter{title}
\end{document}