Category:Wikipedia style guidelines is the peak style guide category for pages related to standards for the design and writing of Wikipedia articles. A page may be considered a style guideline if it a) is actually a formal Wikipedia guideline, and b) is intended to help keep the formatting, grammar, and style of Wikipedia's articles consistent.
Wikipedia guidelines (or sections thereof) that are not part of the Manual of Style, but nonetheless contain style advice – for example Wikipedia:Citing sources – may be categorized here automatically by using
((Subcat guideline|style guideline)) at the top of the page or section, or by just including the category at the bottom of that section.
Style advice pages by wikiprojects are essays, and are categorized at Category:WikiProject style advice, unless and until they are formally adopted into the MoS as Wikipedia guidelines (see below).
Please post a message at WT:Manual of Style if you are thinking of adding a page to this category. We encourage people to write style advice on new topics, if advice is lacking and disputes have been erupting, but most style issues we need to cover are already covered, and if a new one arises, group efforts work best, not individual essays; most topical style guidelines were developed first as wikiproject style advice essays. If you have created a new topical wikiproject and decided that creating a style guide for articles with its scope is the next step, you are probably mistaken, or Wikipedia would have many hundreds of such pages instead of a few dozen.
New proposals for MoS (and therefore guideline) status should take into account the need to rationalise the category, reducing overlap, duplication, and conflicts between advice in WP's style guides. Proposed style guidelines are in Category:Wikipedia style guideline proposals. Consensus should be demonstrated for promotion to MoS status, through a proposal at Wikipedia talk:Manual of Style or at Wikipedia:Village pump/Proposals.
For informal style advice maintained in user space, see Category:User essays on style.