Line Numbers and Double Spacing: The Formatting Rules That Stall Review.

Most journals ask for two things inside the manuscript file before anyone reads the science: continuous line numbers that run the length of the document, and a set line spacing, most often double. PLOS ONE states both plainly. Its rules say "Manuscript text should be double-spaced" and that you should "Include page numbers and continuous line numbers in the manuscript file." Skip either one and the submission system, or the handling editor, can return the file before review begins.

These are not style preferences. They are processing requirements, and they sit in the same category as the desk reject you can see coming: both are avoidable, and both get decided in the first minute someone opens your file.

Why do journals ask for line numbers and spacing?

So reviewers can point to an exact spot and write in the gaps. Line numbers give every comment an address. Spacing leaves room to mark up the page.

Frontiers in Neurology is explicit about the reason: the document "must contain page and line numbers in order to facilitate the review process." A reviewer who writes "line 214, the sample size contradicts Table 2" is faster and clearer than one who writes "somewhere in the methods." Double spacing does the same job vertically, leaving room between the lines for a reviewer or copyeditor to write. Both rules exist to make the file workable for the people who handle it next, not to test your patience.

How do I add continuous line numbers in Word?

Open the Layout tab, find the Line Numbers menu in the Page Setup group, and choose Continuous. That numbers every line straight through, page one to the end, without restarting.

If your document is split into sections, a landscape table page for instance, select the whole file with Ctrl+A before you apply the setting, or the numbering restarts at each section break. Pick Continuous, not Restart Each Page, which resets the count to one on every page and is almost never what a guideline asks for. In LaTeX, load the lineno package with \usepackage{lineno} and switch it on with \linenumbers in the preamble. Many journal document classes, including the PLOS template, already wire this in, so check before you add it twice.

What does double spacing actually cover?

The whole manuscript, not just the paragraphs. Body text, references, tables, and figure legends all fall under a double-spacing instruction unless the guide carves out an exception. PLOS ONE keeps its rule short: "Manuscript text should be double-spaced."

The reference list is the part people forget. It is manuscript text, so a double-spacing rule covers it. Set spacing once for the entire file rather than fixing it paragraph by paragraph: select all, then set line spacing to 2.0 (in Word, the Line and Paragraph Spacing button on the Home tab, or the Paragraph dialog). Then read the specific guide, because the default is not universal. Frontiers in Neurology reverses the common expectation and asks that "the entire document should be single-spaced." Drop a double-spaced template into that submission and you have a mismatch the moment it lands. One journal's correct file is another journal's formatting error, which is why reusing last submission's settings quietly causes problems.

Which file formats do submission systems accept?

Editable source files, not flattened PDFs. For initial submission PLOS ONE wants the manuscript as Word (.docx) or LaTeX, and accepts DOC, DOCX, or RTF. It does not take a single PDF as the manuscript itself.

A PDF locks the text in place. The system cannot read your line numbers as data, run its similarity checks, or let you revise in the same file later. LaTeX authors are the exception with a twist: you upload a compiled PDF for the review stage, but the .tex source files are required if the paper is accepted. Two smaller traps catch people at the last step. Word files should not be locked or password-protected, and figures usually upload as separate files rather than sitting embedded in the text. If the system rejects your upload without a clear message, a protected document is a common cause.

A two-minute check before you upload

Open the finished file and confirm four things. Line numbers run continuously from page one to the last page, with no restart at a section break. The spacing matches what this journal asks, not what the last one asked. The file is an editable .docx or .tex, not a PDF export of your manuscript. Nothing is locked or password-protected.

None of this touches your argument or your data. It is the layer underneath, the part a submission system reads mechanically before a human ever weighs the work. Getting it right is the difference between a file that reaches a reviewer and one that bounces back with a form message. If you want a fuller pass over every field and statement a journal expects, work through a first-submission checklist before you start the upload, and read up on what a desk rejection without review actually means so you can tell a formatting bounce from a scientific one. A manuscript review pass checks your file against a target journal's rules before you submit, which is one way to catch a spacing or line-number miss while it still costs nothing to fix.