A cap (キャップ, kyappu), or capcode, is a special identifier that is meant to prevent impersonation on anonymous bulletin boards. It was added to 2channel as the 'impersonation prevention feature' (成りすまし防止機能, narisumashi bōshi kinō) in 1999, prior to the tripcode system.

Of course, since 'bōshi(feature)' (機能) and 'bōshi(hat)' (帽子) were homophones, the identifiers were later renamed to 'cap'. The term later became 'capcode' because early translations of 2channel's FAQ page had decided to compare the 'cap' and 'trip' to a 'passcode' system in 2003.¹⁾

Most implementations of the capcode system involve using a special string in 'email' field, which instructs the software to replace the name with the proper capcode. The original capcodes used a 'filled-in star' (★) that couldn't be copied or impersonated as the software would change it to an 'outlined star' (☆) instead. Of course, other implementations may utilize different methods, different capcodes, and different checks.

2channel uses the aforementioned 'special string in email field' method. Since the addition of tripcodes, they have been very selective with capcodes, only providing them to management and users who apply to be a reporter on the news-oriented 'plus' boards. In the post-Hiroyuki era, a 'cap stopper' was added in 2014, which let users kill a leaked capcode by using #DEL: before the string in the email field.²⁾

In the past, Hiroyuki had the capcode of fusianasan, which leaked on September 23, 1999, so the capcode would automatically show the user's hostname to discourage further usage. This later became a way of trolling newcomers, called a 'fusianasan trap' (fusianasanトラップ). The post-Hiroyuki 2channel later removed said feature around late 2017, while Hiroyuki's own 2channel clone still has it implemented.

Shitaraba is a website that allows users to create and manage their own boards, so the feature is offered but renamed to 'handle password' (ハンドルパスワード). These capcodes can be registered within the board's management panel, with custom colors set by the board owners, but do note that the special strings must start with a 'number sign' (#) when the designated user enters them into the email field.³⁾

4chan uses the 'special string in options field' method, but overhauled it to check for authentication cookies from the reports panel (reports.4chan.org) to avoid user-level spoofing. Historically, the moderator capcode used to be pubies,⁴⁾ but it was later standardized and changed to capcode_mod, shown in the following chart. This doesn't include the since4pass capcodes anyone can buy since you can look that up.

At one point, the tooltips also revealed internal ID numbers in the secret “Janitor & Moderator Discussion” (/j/) boards, but it was presumably removed since then. Humorously, the source code does contain numerous methods to deter users from impersonating a staff member (e.g. “moot”, “Ep8pui8Vw2”, “##”, “⌘”), though it will simply 'terminate the action' instead of 'banning you for trying' now.

For most Tinyboard-derived software, the name field is used in a similar fashion to tripcodes for capcodes. By default, ## Admin → '## Admin' and “## Mod → '## Mod', and it will simply spit out a secure tripcode if you lack the proper permissions. Of course, the staff isn't bound to the default capcodes as the software allows for custom capcodes, such as '## God' or whatever. This should be very easy to change.

• As mentioned, the homophones don't translate well to non-Japanese langauges.
  • In the Chinese language, it becomes 'jīnéng' (机能, lit. “function”) and 'màozi' (帽子, lit. “hat”).
  • In the Korean language, it becomes 'gineung' (기능, lit. “function”) and 'moja' (모자, lit. “hat”).

• ★ on 2channel's Wiki
• Cap (Electronic Bulletin Board) on Japanese Wikipedia
• Cap (Electronic Bulletin Board) on Niconico Encyclopedia