That’s what she said.

On a more serious note, sometimes I have these weird ideas. My purple Kaweco Al Sport was almost out of ink, and my ADHD (possibly AuDHD) tendencies both pulls me against using up the cartridge in that pen while at the same time wanting to put another in the loop on my Travelers Notebook Passport because what if I run out of ink.

Then I looked at both the cartridge and the pen and wondered if I could fit a spare one on top of the one in it.

Almost. Another 11mm would have made work.

That made me think that all cartridge pens should allow for this.