Seiko’s iconic A239 World Timer – like the all original 1979 example here, with its original bracelet and original box – is a great example of how a dual-layer LCD can give a watch so much more functionality at the press of a button.
 
The As39 was the first watch ever produced with a dual-layer LCD screen, with two LCD panels on top of one another, which it uses to show a world map.  Switching between the two has the effect of completely changing the display; the first panel shows normal time and date, and the second has a world map wherein different time zones can be selected.
 
World Time watches, even Seiko’s own M158 (Pan Am) and A708, usually relied on having a small marker (like the size of the day indicator on many LCD watches) appear at various points around the display to indicate the time in a particular city/country/zone.  The A239 went one better by putting a world map on a separate LCD, split into sections which lined up with the country or time zone being indicated.
 
It offered the option for a daily alarm, as well as a separate world time alarm.  While it was copied/pirated and became the World Time Melody Alarm watch for low-cost distributors Armitron, Caravelle, Zeon and others its popularity never waned.
 
This A239 comes with its original stainless-steel bracelet and box, nylon NATO strap, Pelican travel case, and springbar tool – a manual for the A239 and full details of functions can be found at https://www.manualslib.com/manual/1050580/Seiko-Quartz-A239.html