One topic per thread

From Carls wiki

The idea that email subject lines are meaningful carriers of information, and hence that emails concerning something new should be given a different subject.

Fictional example:

Someone else (>), Carl (>>):
> >
> > [...]
> >
> > So that's why I think BASIC programming is a felony, not a crime.
>
> Hey, are you going to the concert tomorrow? I was thinking maybe we could
> meet up and go there together.

It appears that there is a correlation between adhering to this idea and being a frequent user of email. The people who don't consider separating different topics into different threads seem to be the same ones who tend to give emails a subject like "Hi!" or "Hello Carl", effectively treating it as the greeting line in a physical letter.