Headings and subheadings can help organize and structure your writing. In general, longer and more complex works warrant more of them than shorter ones. Avoid overusing headings in short projects; they should never be used to compensate for poor structure or to explain an underdeveloped idea.
When headings are called for in your writing project, observe the basic guidelines below.
The paper or chapter title is the first level of heading, and it must be the most prominent.
Headings should be styled in descending order of prominence. After the first level, the other headings are subheadings—that is, they are subordinate. Font styling and size are used to signal prominence. In general, a boldface, larger font indicates prominence; a smaller font, italics, and lack of bold can be used to signal subordination. For readability, don’t go overboard: avoid using all capital letters for headings (in some cases, small capitals may be acceptable):
Note that word-processing software often has built-in heading styles.
Consistency in the styling of headings and subheadings is key to signaling to readers the structure of a research project. That is, each level 1 heading should appear in the same style and size, as should each level 2 heading, and so on. Generally avoid numbers and letters to designate heads unless you are working in a discipline where doing so is conventional. Note that a heading labeled “1” requires a subsequent heading labeled “2,” and a heading labeled “a” requires a subsequent heading labeled “b.”
In a project that is not professionally designed and published, headings should be flush with the left margin, to avoid confusion with block quotations. (The exception is the paper or chapter title, which is centered in MLA style.)
For readability, it is helpful to include a line space above and below a heading, as shown in this post.
No internal heading level should have only one instance. For example, if you have one level 1 heading, you need to have a second level 1 heading. (The exceptions are the paper or chapter title and the headings for notes and the list of works cited.) You should also generally have text under each heading.
Capitalize headings like the titles of works, as explained in section 1.2 of the MLA Handbook.
The shorter, the better.