Now we're cookin'.

The C joint is an accessory mounting system using a rather simple 3 mm bar and c-shaped clip. First used in the Revenge of the Fallen/N.E.S.T. Global Alliance series on a few items in 2009, by the time of 2010's gold-packaging Transformers line and Generations, the C joint has seen much more widespread use.

The system has been given the internal nickname "Miyake joint" by TakaraTomy employees, named after its designer Tomoya Miyake,^[1] who first used it on the Revenge of the Fallen Recon Ironhide toy.

Toys with C joints

Revenge of the Fallen

Transformers (2010)

Generations

United

Dark of the Moon

Human Alliance

Cyberverse

• All Cyberverse figures have at least one hand that is a clip. All Cyberverse weapon platforms/playsets have the usual smattering of clips and bars. They also all have a uniform placement of clips and bars on the sides of one of their modes, allowing them to connect to one another.

Titans Return

The Last Knight

Tiny Turbo Changers

"Give me a weapon, sir. Please."

Almost all of the Tiny Turbo Changers feature 3 mm fists for their robot modes. This means that they can cling to any bot or weapon with 3 mm bars, making for either some hilariously over-armed tiny robots... or some comedy antics with the larger bots.

Notes

• LEGO has long made use of 3 mm bars and clips as well. In practice, LEGO's standard is just the slightest bit larger than Hasbro's, so LEGO clips with even a little wear will only loosely grip C joint bars. Still, LEGO minifigures can cling to your 3 mm-bar-equipped Transformers, and their vehicles (assuming they incorporate the appropriate parts) can mount 3 mm clip weapons.

• Naturally, this means that similar-construction Kre-O parts should work as well.

• While C joints per se were not developed until the Revenge of the Fallen toyline, many previously made toys have similarly sized bars that can be used to attach C joint weaponry. At least one example dates to the earliest days of the franchise. The Action Master weapons' handles are all C joint-compatible pegs, though the general lack of clips on the robots themselves means this compatibility has somewhat limited practical application for modern figures. Still, you could give the Action Master partners some extra weaponry.
• ...but be careful about it. As with 5 mm posts, a bar that appears to be 3 mm may not quite be 3 mm. TFWiki.net will not be held responsible if you break the clips on your weapons trying to force them onto bars that are too big.
• Because clips are attached to the right side of the Ark, it is technically the largest C-joint accessory yet made.

References

1. ↑ http://www.tfw2005.com/boards/transformers-news-rumors/359804-united-reveal-shield-jazz-development-secret-story.html