What is house style, and which style guides matter?
House style is a publication's set of decisions about matters where more than one form is correct — so that everything it publishes is internally consistent, rather than reflecting each writer's preferences.
What it covers. Not grammar, which is largely not optional, but the genuinely variable:
Spelling choices — -ise or -ize, judgement or judgment.
Punctuation conventions — the serial comma, single or double quotation marks, dashes with or without spaces.
Capitalisation of job titles, headings and proper nouns.
Numbers — when to spell out, how to render ranges, dates and times.
Abbreviations, and whether full points are used.
Preferred forms for contested or sensitive terminology, which is frequently the most actively maintained part.
Citation format.
Why it exists. Consistency is invisible when present and conspicuous when absent. A reader who notices that quotation marks change between articles has been distracted from the content, which is the failure it prevents. It also removes the need to make the same decision repeatedly.
The major guides:
New Hart's Rules and the Oxford Style Manual — the British reference for editorial style.
The Chicago Manual of Style — dominant in American book publishing and academic work.
AP Stylebook — American journalism.
The Guardian and Observer style guide and The Economist Style Guide — both published and widely consulted beyond their own newsrooms.
MHRA, MLA, APA and Harvard — academic citation and formatting, varying by discipline.
Government and organisational guides, such as the GOV.UK style guide, which is unusually good on plain English.
The practical points:
A style guide is a decision record, not a rule book. Where guides disagree, neither is wrong — they made different choices.
Follow the one you are asked to follow, and do not argue with house style on submitted work; copy-editors will apply it regardless.
Internal consistency matters more than which convention you pick, for anything with no imposed guide.
Keep your own style sheet for a long project, recording decisions as you make them.