Commenting
You can post comments on any reddit link, and read comments that other people have posted. You can even vote on them! Just like links, comments will rise and fall in the rankings as people vote on them.
Reading
reddit's comments are threaded -- when you reply to someone's comment, it gets indented below it. You can collapse an entire comment tree by clicking the [-] on the first line of the parent comment.
Posting
When posting a comment, you're not just limited to plain text. Here's a handy reference table that shows how to use the most common markup:
This table can also be accessed from any comment box by clicking the "help" link.
For more advanced syntax options, check out the Official Markdown Syntax.
Or this popular user-created Markdown primer.
Pitfalls and workarounds
In Markdown, if you put a number followed by a period at the beginning of a line, it assumes that you are making a numbered list and always starts the list at one. If, for example, you tried to write "2009. What a year," it would come out as "1. What a year." You can tell Markdown to leave the number alone by putting a backslash in front of the period: "2009\. What a year."
Individual line breaks are ignored, so if you type:
foo foo foo foo foo foo foo foo foo
bar bar bar bar bar bar bar bar bar
...you get:
foo foo foo foo foo foo foo foo foo bar bar bar bar bar bar bar bar bar
If you actually want the line break to take effect, put two spaces before it. In other words, end the first line with a pair of spaces.
Wikipedia
Links to Wikipedia, and other websites with the same URL pattern, can be problematic when they end in ')'. For example:
http://en.wikipedia.org/wiki/Pica_(disorder)
Many pages on Wikipedia often end with a word in parentheses. When you look at the format to make links in reddit comments, you'll see that [the text goes here, and](the url goes here). Encased in the [] is the text everyone sees, and inside the () is the URL of where you are linking to. Let's try linking to this page with reddit's linking format:
[Pica (disorder)](http://en.wikipedia.org/wiki/Pica_(disorder))
What happened? reddit's code reads the URL as "http://en.wikipedia.org/wiki/Pica_(disorder", and takes the first right parenthesis to enclose the link, rather than have it as part of the URL. Then, an extra ')' is appended after the entire link format. Basically, instead of linking to
http://en.wikipedia.org/wiki/Pica_(disorder),
there is a link to
However, there is a workaround for this. Instead of the original URL, try typing this:
What does this do? Essentially, the %28 and %29 bits turn into left and right parentheses as a URL. Now, instead of reddit code seeing two right parentheses, it sees ('insert text here'), so that the whole URL will be in the link instead of only part of it.
The final result: [Pica (Disorder)](http://en.wikipedia.org/wiki/Pica_%28disorder%29)
