Explanatory essay about the the Wikipedia:Reliable sources guideline
Reliable sources must be able to be verified. This does not mean that any particular person at any given moment must be capable of verifying them.
Verifiable sources may have time restrictions (only accessible between 10am and 4pm in a particular time zone).
Verifiable sources may have location restrictions (only available at one archive, museum, repository, or only available within a certain country or geographical area).
Verifiable sources may have cost restrictions (the purchase of a book, journal article, magazine, newspaper, or the Interlibrary Loans or Document Delivery costs associated with them, access to a museum costs, costs of entry to paid archival services).
Verifiable sources may have technical or personal restrictions (written in languages other than English, on websites that restrict access or require certain software, available on a type of media that requires the reader to have a certain type of technological appliance to access it).
The costs or difficulties of verifying a source do not impact its reliability, so long as it is possible for someone to verify it within a reasonable time.
Where a source is difficult to verify, or in a language other than English, many editors appreciate the courtesy of supplying the relevant paragraph and ensuring it can be read by English language readers. When sources of equal quality are available, the ease of access may be preferred. But if sources of higher quality are difficult to verify, that difficulty alone is not a reason to disregard such sources or replace them with lower-quality ones.
Where a source is difficult to verify, and legal to duplicate or duplicate portions of, produce impressions of, or otherwise make a recording that will allow other editors to verify off the copy, it is a reasonable expectation, but not mandatory, that this be done to assist other Wikipedia editors in verification. Books in the public domain may be uploaded to Wikisource.
It is helpful for readers to know which sources they can and can't access to evaluate their reliability directly.
The {{citation}} template has options to indicate access-level, e.g., "url-access".
The {{open access}} template similarly highlights access level with an icon.
The {{Link note}} template can be used to indicate access requirements: {{link note|note=Registration required}}
The text after "note=" is displayed in the text in parentheses, e.g. (Registration required). The template can add pages to a category to track such links. Various other relevant templates, such as {{Subscription required}}, are listed in Template:Link note#See also.
