How do comments work?
Comments exist so that users can talk about questions and answers without posting new answers that do not actually answer their parent questions. Comments are often used to ask for clarification on, suggest corrections to and provide meta-information about posts.
Comments are intentionally short, having maximum length of 600 characters, and allow only limited markup. URLs in comments automatically become hyperlinks. Each user may post only one comment every 15 seconds.
Comments are disposable: unlike posts, there's no revision history, and they can be deleted without warning by their authors, by moderators, and in response to flags.
Who can post comments?
All users may leave comments on their own posts and any answers given to their own questions. Users with at least 50 reputation may comment on any post. (There is no reputation requirement to post comments on MSO only; doing so elsewhere, including on other meta sites, requires 50 reputation.)
See also: http://meta.stackoverflow.com/privileges/comment
Who can edit comments?
Users may edit their own comments any number of times for five minutes after they are first posted. Edited comments are marked with a little pencil icon, and a mouseover tooltip over that icon will provide a count of how many times the comment was edited, like this:
Moderators can edit any comment at any time. Moderator comment edits are logged and visible to other moderators, but no revision history is kept. Such edits will also show the pencil icon.
In all other situations, comments cannot be edited. However, in lieu of editing, they may be deleted and resubmitted.
How can I format and link in comments?
Comments can be formatted with a subset of Markdown: bold (
**bold**), italic (
*italic*), bold italic (
***bold italic*** or
___bold italic___) and
`code`) are allowed. It is not generally possible to insert line breaks in comments, though some hacky workarounds exist.
URLs will be automatically converted into actual links. Links are also allowed by using the Markdown syntax
[link text](http://myurl), and there are certain "magic links" that will be converted to real links automatically.
Spaces are not allowed in the comments URLs (even markdown), for example this:
[URL](http://google.com/?q=stack overflow) will not generate a link. It needs to be encoded like this:
See also: http://meta.stackoverflow.com/editing-help#comment-formatting
Also see: http://meta.stackexchange.com/a/79981/187824
Who can delete comments?
A user may delete one of his own comments by clicking on the (x) icon that appears to the right of the comment's timestamp when the comment is moused over. A user can delete only one comment every five seconds.
Comments that are flagged by multiple users are deleted automatically. The number of flags needed is based on the comment's score. It currently takes "
3 + (Score / 3)" flags to delete a comment.
Moderators can delete any comment, or purge all comments from a post.
When should comments be deleted?
Comments are temporary "Post-It" notes left on a question or answer. You should not expect them to be around forever: Once a clarification has been made, an edit added to the post to include new information, or the issue in the comment is otherwise resolved, it is subject to deletion. In reality, many obsolete or chatty comments remain untouched due to the high volume of comments posted, but this does not mean that they can't or shouldn't be deleted in the future.
What are automatic comments?
In several cases, the system will automatically add comments to posts on your behalf. You still own the comment, and it will appear the same as if you had manually posted the comment. You may delete the comment if you desire, but this is discouraged as it is helpful to the original poster: the automatic comments provide information on how to improve the post or why it is not appropriate.
When someone flags or votes to close a question as an exact duplicate, the system automatically posts a comment to that question which reads "possible duplicate of -link-", as long as no such link has already been posted in a comment. The comment will be automatically deleted if the question is closed as a duplicate.
When voting to close a question, the user can choose Off Topic and choose "Other", adding a custom reason. When doing that, automatic comment will be added on his behalf with the exact text he/she wrote, usually something like "This question appears to be off-topic because it is about [custom text here]". That user can delete or edit the comment within 5 minutes as any comment he posted, and that kind of automatic comment is not deleted when the question is put on hold. Note that this option is not present when flagging.
Note that when flagging a question as off topic or a duplicate, if the user does not have the comment everywhere privilege, these options will not be present in the flag dialog.
When choosing the "Recommend Deletion" action in the Very Low Quality review queue, an automatic comment will be added to the actioned post, unless the user selects the "no comment necessary" option. If the selected automatic comment is already present on the post, the existing comment will receive an upvote and a duplicate comment will not be added.
How can I link to comments?
The timestamp at the end of each comment is a permanent link to that comment. This behavior has existed since January 2012. Before then, it was not officially possible to directly link (permalink) to comments, though an unsupported workaround existed.