back up (verb) / backup (noun): Style Guide Spelling Decisions (AP, Chicago, MLA, APA)

How major style guides rule on "back up (verb) / backup (noun)"

Verb is open ('back up your files'); noun and adjective are closed ('a backup copy').

The disagreement on "back up (verb) / backup (noun)" is an example of phrasal verbs (open) vs derived nouns (closed): the category of style-guide differences that most often confuses copy editors and creates inconsistency across long documents. Below is a guide-by-guide breakdown, drawn directly from the published editions cited.

What the divergence actually means

Unlike many compound-modernization decisions where style guides eventually converge, "back up (verb) / backup (noun)" continues to show genuine divergence between major guides. The AP Stylebook treats this as a settled call; Chicago Manual leaves more flexibility; and Merriam-Webster, as a descriptive dictionary, records both forms. Source: Chicago Manual of Style, 17th Edition

The APA Publication Manual aligns with AP and Chicago on "back up (verb) / backup (noun)", though APA generally addresses spelling questions only when they intersect with statistical reporting or technical psychology vocabulary. Where the manual is silent, APA defers to Merriam-Webster as its standard reference. Source: APA Publication Manual, 7th Edition

Merriam-Webster lists "back up (v.), backup (n., adj.)", which serves as the lexicographic baseline for U.S. style decisions. Because Merriam-Webster's entries reflect aggregated published usage rather than editorial preference, when a guide says "follow Merriam-Webster", as APA does, that effectively delegates the call to whichever spelling has dominated the published corpus. Source: Merriam-Webster Dictionary

Practical guidance for editors

For working writers, the practical rule is straightforward: in journalism, follow AP; in academic writing in the humanities, follow MLA or Chicago; in social-science publishing, follow APA; in book publishing, follow Chicago. When no house style applies, Merriam-Webster's main entry is the safest default. The differences across these guides on "back up (verb) / backup (noun)" reflect not disagreement about correctness but different audiences and editorial pressures.

Cross-references

For the dictionary entry, frequency data, and pronunciation of back, see the main word page. For other style-guide spelling decisions in this category, browse all style-guide spelling rulings.