I have not been able to find the answer to this question anywhere here - maybe it's too elementary.

Do any of the main Japanese pens take each others carts? Or do I have to carry carts for each make of pen.

Solitaire

I don't know about Platinum but Sailor and Pilot use different size mouths on their cartridges. The Pilot mouth is larger than the Sailor. Here's a very poor photo (Sailor on the left, Pilot on the right).

Pilot, Platnium and Sailor each have proprietary cartridges for their pens. Platnium carts have a ball bearing inside to break surface tension.

Thank you very much.

So that means
1. Pilot/Namiki
2. Platinum/Nakaya
3. Sailor

Each have different cartridges and converters
That takes care of my own few Japanese pens so thanks again

Solitaire