TDN Notation (TDN表記, Tadano Hyōki) is a writing system that would be popularized by Inmu after it was used to obfuscate the identities of those involved in Tadano's scandal, though it's commonly believed that the system predates the 2000s. It is somewhat similar to disemvoweling.

Historically, the original rule for TDN Notation was 'one alphabet letter per kanji', which brought us the legacy names of 'Tadano' (多田野) becoming 'TDN', 'Daibō' (大坊) becoming 'DB', and 'Hatano' (羽田野) becoming 'HTN'. However, the rule fell apart since 'Tanioka' (谷岡) would've been 'TO' and that would've confused people, so it was then adjusted to 'one alphabet letter per kana' instead, bringing us 'TNOK'.

At some point, the naming scheme caught on, as people were already saying things like AKB48 or PKMN, so it wasn't out of the question to start using FKMT or KYM. In addition, the naming scheme has also been used by shippers, as most Japanese ship names follow a four kana clipped compound format, making it possible to discuss ships without alerting their shippers.

1. Take a Japanese name, or convert a non-Japanese name, and reduce it to its kana.
2. Convert to wāpuro rōmaji¹⁾ and follow the 'one alphabet letter per kana' rule.
   • The exception seems to be names that either have a common romanization or an English name, which include 'Darvish' (ダルビッシュ), 'jigoku' (地獄), 'coffee' (コーヒー), etc.
3. If the name is taken, alter the notation (e.g. use another system) in a way that makes sense.

• TDN Notation on Pixiv Encyclopedia
• TDN Notation on Niconico Encyclopedia
• TDN Notation on Takahiro Karasawa Wiki