\documentclass[a4paper,twoside, symmetric,justified,notoc, nobib]{tufte-book}
\titleclass{\subsubsection}{straight}
\begin{document}
\frontmatter
\pagenumbering{roman}
\setcounter{secnumdepth}{-1}
\mainmatter
\section{CHAPTER 1}
\subsection{Introduction}
\subsubsection{An additional sub-heading within introduction }
\end{document}
Using the tufte-book class, I've been trying to define \subsubheading as I'm finding that \subheading alone isn't giving me enough flexibility for nesting content.
I found this question here on tex.SE which appears to want a similar thing, but in my case I'm not interested in the additional formatting (I don't wish to define the formatting from scratch, but rather just inherit formatting).
My aim is to just copy the formatting for \subsection and use this for \subsubsection.
My solution to this was to define a new titleclass such that \titleclass{\subsubsection}{straight} (this was suggested in several other posts and is covered in the LaTeX companion as away of introducing new sections/titles), but what I can't do is make it format correctly. Whilst in the ToC everything looks fine, on the page, the {straight} command gives bold, upright text.
Is there a way of making this new title class inherit the style of the old \subsection?
