What is a historical lock?
A historical lock is a mechanism by which moderators can mark posts as historical artifacts. Questions which are historically locked feature the following post notice:
locked by Moderator ♦ Mar 16 at 20:01
This question exists because it has
historical significance, but it is not considered a good, on-topic
question for this site, so please do not use it as evidence that you
can ask similar questions here. This question and its answers are
frozen and cannot be changed. More info: help center.
This is an example of a question with a historical lock: Changes to syntax highlighting
What is the purpose of a historical lock?
A historical lock preserves content that was very popular when it was originally posted, but is now off-topic or otherwise out of scope for the site it is posted on. Historically locking a post ends the debate over whether a question should be kept on the site or deleted, and is often the final state of a question that has been deleted and undeleted more than once, or subject to a close or delete war.
How are questions affected by historical locking?
In addition to the post notice being prominently displayed, posts which are historically locked are "frozen in time": they cannot be voted on, flagged, answered, edited or commented (though they can be favorited, and ♦ moderators can edit and comment; moderator edits to historically locked posts do not bump the posts). The visual appearance of the entire post is altered by removing the voting arrows from the question and all answers.
Additionally, on main (non-meta) sites, historically locked questions are omitted from normal question lists (those on the home page, /questions, and the various per-tag lists), but can still be found by searching for words in the post or title (via either site-search or Google, etc) or by using the search operator
When is it appropriate to lock a question for historical reasons?
Questions can be historically locked when:
- The post does not meet the current guidelines for a good, on-topic question, and
- The post is stellar, in spite of its off-topic nature, and
- There are a large number of views, upvotes and inbound links on the post, and
- The post is contentious; e.g., it has been closed and reopened at least once, or deleted and undeleted at least once.
When is it not appropriate to historically lock a question?
Questions should not be historically locked if they are being actively maintained or have little or no redeeming value.
A good rule of thumb: If the question does not minimally meet Jeff Atwood's 3rd rule in the "We Hate Fun Here" blog post, it's probably not a good candidate for historical locking. The third rule is:
Does this question teach me anything that could make me better at my
job? Can I learn something from it?
How do I request a historical lock on a question?
Flag the question for moderator attention and explain why you think the post should be historically locked. A moderator will evaluate the question using the criteria outlined above and will either lock the question or decline your flag with an explanation.
Alternatively, if you want to try and gain more community support for the question, or contest the moderator decision, you can post a question on the per-site meta.
What should I do if I notice a serious problem with a historically-locked question or one of its answers?
The system prevents you from being able to flag historically-locked posts; this is because these posts have tended to get lots of frivolous flags in the past. Therefore, if you notice a serious problem with a historically-locked post (e.g. a broken link that has since started serving spam or malware, or content that is blatantly offensive), post a question on the per-site meta explaining the exact problem. (If the locked question is here on Meta Stack Exchange itself, post your request here on this site with the tag meta.)
You should reserve this for serious issues like the ones above, rather than minor issues like grammar fixes or things that go against the rationale for historical locks.