[Tex/LaTex] titlesec error “undefined control sequence”

errors

I'm new to titlesec and LaTeX in general, trying to use it to achieve chapters in a {book} class that are medium-sized and on the same line, viz:

Chapter 1: An Unlikely Event

Here's a minimized example:

\documentclass[english,12pt,oneside]{book}
\usepackage{titlesec}
\titleformat{\chapter}[hang]{\normalfont\medium\bfseries}{\chaptertitlename\ \thechapter:}{1em}{}
\begin{document}

\chapter{An Unlikely Event}

\end{document}

The error that returns is:

ERROR: Undefined control sequence.

--- TeX said ---
<argument> \normalfont \medium 
                               \bfseries 
l.77 \chapter{An Unlikely Event}

--- HELP ---
TeX encountered an unknown command name. You probably misspelled the
name ...

I think this is probably a simple error on my part, but I'm too young to know what it is.

Best Answer

you have used the command \medium, which doesn't exist. what you probably meant is \mdseries, which is the default.

since you responded in a comment that what you really want to do is change the size of the font, the command for that is \normalsize.

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