On-Topic: Anything that good hackers would find interesting. That includes more than hacking and startups. If you had to reduce it to a sentence, the answer might be: anything that gratifies one's intellectual curiosity.

Off-Topic: Most stories about politics, or crime, or sports, unless they're evidence of some interesting new phenomenon. Videos of pratfalls or disasters, or cute animal pictures. If they'd cover it on TV news, it's probably off-topic.

In Submissions

Please don't do things to make titles stand out, like using uppercase or exclamation points, or adding a parenthetical remark saying how great an article is. It's implicit in submitting something that you think it's important.

If you submit a link to a video or pdf, please warn us by appending [video] or [pdf] to the title.

Please submit the original source. If a blog post reports on something they found on another site, submit the latter.

If the original title includes the name of the site, please take it out, because the site name will be displayed after the link anyway.

If the original title begins with a number or number + gratuitous adjective, we'd appreciate it if you'd crop it. E.g. translate "10 Ways To Do X" to "How To Do X," and "14 Amazing Ys" to "Ys." Exception: when the number is meaningful, e.g. "The 5 Platonic Solids."

Otherwise please use the original title, unless it is misleading or linkbait.

Don't abuse the text field in the submission form to add commentary to links. The text field is for starting discussions. If you're submitting a link, put it in the url field. If you want to add initial commentary on the link, write a blog post about it and submit that instead.

In titles, please don't describe things by their relation to YC unless they're actually associated with YC.

Please don't post on HN to ask or tell us something (e.g. to ask us questions about Y Combinator, or to ask or complain about moderation). If you want to say something to us, please send it to hn@ycombinator.com.

Similarly, please don't use HN posts to ask YC-funded companies questions that you could ask by emailing them.

Please don't submit so many links at once that the new page is dominated by your submissions.

In Comments

Be civil. Don't say things you wouldn't say in a face to face conversation.

When disagreeing, please reply to the argument instead of calling names. E.g. "That is an idiotic thing to say; 1 + 1 is 2, not 3" can be shortened to "1 + 1 is 2, not 3."

Please avoid introducing classic flamewar topics unless you have something genuinely new to say about them.

Please don't sign comments, especially with your url. They're already signed with your username. If other users want to learn more about you, they can click on it to see your profile.

Please don't use uppercase for emphasis. If you want to emphasize a word or phrase, put *asterisks* around it and it will get italicized.

Please don't submit comments complaining that a submission is inappropriate for the site. If you think something is spam or offtopic, flag it by going to its page and clicking on the "flag" link. (Not all users will see this; there is a karma threshold.) If you flag something, please don't also comment that you did.

If your account is less than a year old, please don't submit comments saying that HN is turning into Reddit. (It's a common semi-noob illusion.)

Resist complaining about being downmodded. It never does any good, and it makes boring reading.

Please don't bait other users by inviting them to downmod you.