In-reply-to

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in-reply-to is the h-entry property to use in a post that is a reply to another post. The reply post must hyperlink to the original post with class="u-in-reply-to".

The in-reply-to property supersedes rel-in-reply-to. For more information, see rel-values.

Additionally:

The in-reply-to link is typically inside the reply-context of a reply post
The microformats2 u-in-reply-to h-cite class names can also be used to embed a reply-context with more details beyond the URL

How to consume in-reply-to

Summary: collect an ordered list of unique in-reply-to URLs by checking and adding from:

the first h-entry for "in-reply-to" URLs (including embedded)

algorithm

Given a URL of a post, to determine the URL(s) it’s in-reply-to:

Let in-reply-to-urls be an empty list
Parse the post's HTML for microformats2. If there are any h-entries, get the first one
- If the h-entry has an in-reply-to property, for each item in its array of values:
  - If the item is a string, append it to in-reply-to-urls if not already there
  - If the item is an embedded microformat, check if it has a url property. If it does, append the first url property value to in-reply-to-urls if not already there
  - Otherwise ignore the item
in-reply-to-urls is now a list of all the URLs the parsed URL is in-reply-to

You may use a different algorithm if it produces exactly the same results. E.g. you could skip checking "if not already there" conditions, and instead at the end perform a step of "* Remove duplicate values from in-reply-to-urls" - as long as you removed latter duplicates, and kept first occurrences in the list.

Usage Examples

tantek uses u-in-reply-to along with h-cite to mark-up reply notes. E.g. [1]

FAQ

Must an in-reply-to link be inside an h-entry

Q: Does an in-reply-to link have to be inside an h-entry?

A: Yes. An in-reply-to link must be inside the h-entry with class name of u-in-reply-to so that it can be recognized as a URL property of the h-entry.

Past Research

See:

http://microformats.org/wiki/rel-in-reply-to

For the research and development of the u-in-reply-to property and rel=in-reply-to.