Groups.io uses this algorithm to thread messages into topics:

• If the message contains threading information (a References and/or In-Reply-To field in the message header that matches the ID of a message in an existing topic), and the subject line matches the subject line of the referenced message, add that message to the topic.
• When a message does not contain any threading information:
  • If the message was posted on the group’s website through the New Topic function (that is, it was not entered as a reply), assume it is the start of a new topic.
  • If the subject starts with Re:, look for a message with a matching subject that was posted within the last 30 days.
  • If the subject does not start with Re:, look for a message with a matching subject that was posted within the last 2 days.

When comparing subject lines, the algorithm ignores:

• Prefixes such as Re: that often are automatically added to replies.
• The group’s subject tag.
• Hashtags that are added to or removed from the subject line.

Note: Group owners and moderators can merge separate topics into one topic or split an existing topic into separate topics.