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:

\titleformat{\chapter}[hang]{\normalfont\medium\bfseries}{\chaptertitlename\ \thechapter:}{1em}{}

\chapter{An Unlikely Event}


The error that returns is:

ERROR: Undefined control sequence.

--- TeX said ---
<argument> \normalfont \medium 
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, 2015 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. May 26, 2015 at 19:35
  • \medium is a command that doesn't exist. Probably you meant mdseries.
    – Bernard
    May 26, 2015 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/… May 26, 2015 at 19:45
  • @StevenArntson -- then what you probably meant was \normalsize. May 26, 2015 at 19:53

1 Answer 1


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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