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Most available 12 on top mechanical and quartz 24 hour watches.
Over the years I mainly wore 24 on top. I do own a vintage Tisot Seastar with 12 on top but since 12 on top is more rare than 24 on top the rest of my collection all had 24 on top.
After I bought a Glycine Airman Vintage V in 2005, 12 on top became my favorite setting.
Recently I even switch from cosmonaute to chrono-matic 24h.
If you click this link you can see the 12 versus 24 poll this site started in 2000.
With 65% in favour of 24 on top it's clear why most watch companies make 24 on top.
As I wrote before, for me a 24 hour watch stands for the earth spinning.
Due to this spin the sun "travels" it's imaginairy path through the sky and at night it travels down under to appear again in the morning. Exactly this is what a 12 on top depicts.
When I changed from 24 to 12 on top I immediately noticed that I adapted to this sun-path with more ease than when 24 is on top.
I still think that no matter who's on top 6, 12, 18, 24 or 00 .... a watch needs to be purist.
12 on top in my case really makes=dipicts my day.