Is it bad style if a section has exactly one subsection?

Question

In a discussion on why subsubsections are evil, egreg comments:

If you have subsubsections, you must have at least two of them in a subsection; you must have at least two subsections and, of course, at least two sections. Draw the tree and you'll understand.

When I'm writing long texts, such as for my thesis, I might have sections that contain only one subsection. For example:

5.3 Topic X

... bla intro bla ...

5.3.1 Special Case in Topic X

... bla detail bla ...

Is it considered bad style to have 5.3.1, but no 5.3.2? Should I rather put the bla intro bla in a 5.3.1 Introduction and have the special case in 5.3.2? What are the general approaches here, what is considered good or bad style, and why?

Answer 1

Having a section with a single subsection is usually considered bad style: most style guides, academic or not, advise against it. In particular, the Chicago Manual of Style advises against it.

Regarding the reasons why this is considered bad practice, I'll quote Wikipedia:

Single Subsections
Just as your English teacher told you, if section 2 has a subsection 2.1, there'd better be a section 2.2 as well. If you see a section with a single subsection, you have three choices:

• If there's a lot of text in the section, followed by the subsection, you ought to be able to carve out a good subsection from the initial material, or even two, to create multiple subsections.

• If most of the section's material is in the subsection, you may not need a subsection. Just combine the two.

• If the content of at the top of the section is short and substantially different from what's in the subsection, you might be able to promote the subsection (for example, change the heading from level 3 to level 2). On other hand, if the subsection covers something relatively unimportant, then don't promote it to a level 2 (top-level) heading.

Other sources, both style guides and writing advice, say the same thing: 1, 2, …

Answer 2

IMHO, it is a bad style in most contexts, and I will try to explain why, on your example:

5.3 Topic X
blabla

5.3.1 Special Case in Topic X
bleble

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If you write a long work (like a thesis or a chapter of a book), you have to use sub-sectioning, still you should keep a good structure. In your case, I would use

5.3 Topic X

5.3.1 General Approach [or a similar title]
blabla

5.3.2 Special Case [and not "Special Case in Topic X", that's redundant]
bleble

One of the reasonable measures whether you have a good structure of your work is the Table of Contents. Just look at it: If it looks good and truly helps in using the text, then the structure is good.

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If you write an article and this would likely be the only subsection in it, just make it a section:

5 Topic X
blabla

6 Special Case in Topic X
bleble

If it's too short to deserve a section, it's quite likely too short to deserve a subsection, in the case you don't use subsections to keep some general structure of the works.

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(Needed to say, I speak as a mathematician and as a typesetter of a tech journal. I've seen solitaire subsections a lot. Unfortunately it is out of my privileges to remove these in the articles I typeset.)