The Butterfly Dial is an ingenious invention designed to combine both the 12 hour and 24 hour dials into a single 12 hour dial.
It’s much easier to see it in action than try to explain it:
The video can be seen on YouTube.
It’s no substitute for a true 24 hour dial, but it’s a much neater solution than the Cyclos dial, described in the Design page of this site:
24hourtime.info 