# titlesec error “undefined control sequence”

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.

• What is \medium? That is the problem. – azetina May 26 '15 at 19:34
• since you're specifying fonts, and the next thing you ask for is \bfseries, you needn't specify "medium". there is \mdseries, to which is equated \mediumseries, but no \medium -- but since it's redundant anyhow, you can just get rid of it. – barbara beeton May 26 '15 at 19:35
• \medium is a command that doesn't exist. Probably you meant mdseries. – Bernard May 26 '15 at 19:37
• I was trying to specify the size of the font, but I must have misunderstood the advice here: tex.stackexchange.com/questions/59726/… – Steven Arntson May 26 '15 at 19:45
• @StevenArntson -- then what you probably meant was \normalsize. – barbara beeton May 26 '15 at 19:53

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.