The default is singular, e.g.
But I am wondering if it is better to use the plural form, e.g.
Are there any guideline on this?
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There's a simple rule that I follow (it's hard to explain, so I'll give examples):
http://example.com/categories/ <-- This plural form should mean that the page shows a list of all the categories in use on your site.
Similarly, if it's "authors" (as in
http://example.com/authors/), I'd expect the page to show a list (with or without descriptions — is your choice) of authors.
Simply put, you can't use "category" (as in
http://example.com/category/) and list all the categories under it. Kinda sounds meaningless, right? The point is, you never know when you'd have to create a listing as such => it's always best to leave out "plural" words for unless you have a specific/definite purpose (which makes sense).
Also, people expect a URL to be a precise description of the page. For example, in my own case, when the title of an article in Google isn't descriptive enough, I look at the URL of the article to see if it's even close to what I am searching for. Just saying.
http://example.com/category/technology/ tells me that I am going to the technology category on example.com
If I see
http://example.com/categories/technology/ my brain needs to think a little bit, and it will of course (in most cases), come to understand it like this — I am going to a page about technology which is one of the categories on example.com)
Sometimes, URLs are structured to show content hierarchy. Take a look at this site's URL structure, for example (Notice it? Yeah, "questions".):
http://wordpress.stackexchange.com/questions/ takes you to a list of all the questions on WP.SE .
53474 is the post ID. That's it these are the essential parts as far as the system is concerned. The descriptive part of the URL, i.e.
permalink-format-singular-or-plural is meant for search engines, and the visitors coming from search engines (AFAIK).
The content hierarchy here?
Home > Questions > Post (or say, Question) —- this made more sense to the people who built the Stack Exchange network, and so, they went ahead it.
: WP.SE => WordPress.StackExchange.com
There are two reasons to keep the singular:
It is a common practice to use the singular. Don't break it unless you want to make it hard to remember your permalinks. Your readers are on other sites 99% of their time, they shouldn't have to learn something without a really good reward.
Plural forms are harder to translate. When I create custom taxonomies or post types I make the slugs translatable too. Singular forms are usually easier to translate, that may have been the origin for this practice.