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.
| Style guide | Preferred form |
|---|---|
| AP Stylebook (2024–2025 Edition) | back up (v.), backup (n., adj.) |
| Chicago Manual of Style | back up (v.), backup (n., adj.) |
| MLA Handbook | back up (v.), backup (n., adj.) |
| APA Publication Manual | back up (v.), backup (n., adj.) |
| Merriam-Webster Dictionary | back up (v.), backup (n., adj.) |
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
Garner's Modern English Usage classifies the "back up (v.) / backup (n., adj.)" / "backup (verb)" pair on its Language-Change Index, a five-stage scale measuring whether a once-disputed form has been accepted into Standard English. For phrasal verbs (open) vs derived nouns (closed), Garner's typically rates the dominant form at Stage 4 ("ubiquitous but objected to by traditionalists") or Stage 5 ("fully accepted"). Source: Garner's Modern English Usage, 5th Edition
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.
Editorial context
In computing, the closed backup thrives as a verb, defying the formal back up / backup distinction endorsed by major guides and persisting in software documentation, command-line utilities, and sysadmin parlance. This stems from 1980s PC-era tools like MS-DOS's Backup command and early hard-drive software, where backup your files to tape became idiomatic for efficiency in technical registers. Even as cloud services like iCloud shifted toward back up by the 2010s, legacy usage lingers in APIs, scripts, and vendor glossaries, witness Oracle and IBM manuals blending forms. Copy editors encounter this in cross-domain texts, where tech contributors default to the solid verb, forcing house-style overrides that risk alienating specialist readers. Published tech corpora, such as the ACL Anthology, reveal backup verb frequencies rivaling the open form, highlighting an industry-specific convention that style consensus struggles to supplant.
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.