At twtxt.net the Twt Subject was invented as an extension to the original Twtxt File Format Specification.

Purpose

Twts in their purest form provide only the mentions mechanism to reply to certain twtxt users. This works well in small, low traffic twtxt communities. However, if there are several ongoing discussions at the same time, a single mention may not be enough for consuming twxt users to clearly identify the exact conversation this twt is considered part of by its author. So twtxt users quickly started to provide more context in parentheses at the beginning of the twt right after any mentions – the so called subject – in the form of:

@<nick1 url1> @<nick2 url2> (re: topic) That's what I think as well.
                            ^^^^^^^^^^^
                            traditional subject

The twt subject provides a mechanism to specify references in twt replies and thus group twts into entire conversations.

Format

The twt subject is the very first contents in parentheses right after any optional mentions in the twt text. The opening and closing parentheses are not part of the subject contents, but rather enclose it. Apart from mentions and whitespace, there must not be any other text preceding the subject or else the parenthesized text must be treated as regular text.

Traditional Human-Readable Topics

Subjects containing only topic references in natural language, such as the example in the Purpose section above, do not have any restrictions. They should be concise, so that users can make sense of them and find the related twts manually themselves. The syntax is:

(topic) text
^^^^^^^
human-readable twt subject

Or:

@<nick url> (topic) text
            ^^^^^^^
            human-readable twt subject

Examples of replies referencing the topic “re: extension spec” (keep in mind these twts are on one physical line, but may be rendered in several ones depending on your font size and screen width):

@<joe https://example.com/twtxt.txt> @<kate https://example.org/twtxt.txt> (re: extension spec) Yes, I agree.
(re: extension spec) But what about…?

Clients may only preserve those kind of subjects as separate entities if they can make use of it, e.g. coloring them differently or showing them in a dedicated subject column when employing a tabular view.

Machine-Parsable Conversation Grouping

To further improve traditional subjects with only references in natural language, the Twt Hash of the first twt starting the conversation should be used in form of a Hash Tag in the twt subject. This machine-parsable version of subjects allows clients to easily group several twts to conversations automatically.

The hash tag may be surrounded with other text, although this is discouraged. There must be exactly one hash tag in the subject. The syntax is:

(#<hash url>) text
^^^^^^^^^^^^^
machine-parsable twt subject

Or:

@<nick url> (#<hash url>) text
            ^^^^^^^^^^^^^
            machine-parsable twt subject

Clients must only use the tag (the twt hash) part of the hash tag rather than its URL when grouping twts to allow for different twtxt.net instance URLs being involved. This way users are able to point the twt hash tag URL to their own instances without running the risk of clients splitting up conversations. It’s also possible to reference different endpoints, such as single twt, tag search or conversation view.

A conversation uses a single twt hash in all the subjects throughout the whole discussion, namely the one from the twt starting the conversation. In order to branch off, the appropriate twt hash can be used in the following subjects to form complex conversation trees rather than just linear flows.

Clients may hide the subjects to use the available space more efficiently for contents.

Examples of replies referencing a twt with hash “abcdefg” (keep in mind these twts are on one physical line, but may be rendered in several ones depending on your font size and screen width):

@<joe https://example.com/twtxt.txt> @<kate https://example.org/twtxt.txt> (#<abcdefg https://example.com/search?tag=abcdefg>) Yes, I agree.
(#<abcdefg https://example.org/conv/abcdefg>) But what about…?
@<joe https://example.com/twtxt.txt> (#<abcdefg https://example.com/search?tag=abcdefg>) @<kate https://example.org/twtxt.txt> Hmm, are you sure? What if…