One of the hallmarks of effective professional communication is order. In the world of business, industry, and government, we want things to stack up neatly, to line up, to be accessible and easily searchable. We like patterns that help orient our brains to presentations of information. That's why, for example, when someone is giving an oral presentation, we really appreciate it when she starts by orienting us to the purpose and structure of her presentation, perhaps by providing and briefly reviewing an outline (in the form of a handout, or maybe on the overhead or data projector).
Outlines display hierarchies of information. Good outlines tell us both what's in a document and how its parts are related to each other. We can see at a glance what the main parts of the document are and what sub-categories fall into each major category. If the author or speaker reveals her principle of organization (as she should), we can even see at a glance the order of importance of the topics, or the order of their difficulty, or their placement on a timeline, etc. These principles of organization are many, and the effective communicator chooses the ones best suited to her purpose and audience.
However, no matter what overarching principles of order are chosen, there is what we might call an "interior principle of order" that applies to every kind of communication, at every level (from the "macro level," the main sections of documents, to the "micro level," individual words within sentences and phrases). This is what we call "parallel form." Let's first consider parallel form as it applies to headings.
Form in Headings
However, even documents graced with lots of headings can be less accessible and helpful than they could be if parallel form were respected.
Consider, for example, the following major heading and its three sub-headings:
Problems with New Line of Laptops
Each of these subheadings, considered separately, is absolutely fine. What's not fine is the lack of parallel form between them. Since each is simply an example of some "problem with the new line of laptops," why shouldn't each point be expressed in parallel form, grammatically and stylistically? Why should the reader be subjected to even the slightest demand to reorient his brain when looking at that list of problems? Answer: there is NO good reason.
if you can edit those subheads to make them parallel in form: Check your
edit against these possible answers:
you don't want to use such long noun phrases for your headings--perhaps
because there's not much info under each category, and the entire spread
of main heading and subheadings is going to appear on one easily accessible
page? Then how about this:
many variations. Whatever form you choose, remember to make headings that
are coordinate in sense parallel in structure. This helps readers recognize
the relationship between blocks of information. And it does wonders for
your document's coherence.