24 hour dial
A 24 hour dial is one with markings on a scale of 24 hours rather than the typical 12.
Markings and orientation
Most 24 hour watches do not feature 24 numbered labels, since this would be very cramped. Rather, they alternate even numbers and odd markers at most. This is typical of 12 hour watches as well, with the "3, 6, 9, 12" markings being very common. Similarly, many 24 hour dials are labeled "6, 12, 18, 24".
There is a great deal of variation as well on the orientation of the dial. Many 24 hour watches place the 24 marker at the top, but others prefer placing the 24 at the bottom, since this better represents the day/night cycle. Still others place 24 on the left or right, with this being the "night" half of the dial.
24 hour watch
A true 24 hour watch has the main dial marked in 24 hour increments rather than a 24-hour subdial, "12 and 12" arrangement, or secondary 24-hour GMT hand. Perhaps the best-known 24 hour watches are the Breitling Cosmonaute and Glycine Airman.
24 hour watches are fairly uncommon, especially from the major brands. One reason for this is the rarity of off the shelf 24 hour movements from ETA and other major providers; most 24 hour watches use in-house movements, repurposed GMT movements, or a custom motion work. Only the ETA 2893-2 was designed to be used as a 24 hour movement.
Another reason for the rarity of 24 hour watches is consumer demand. Many have grown used to the customary locations of hours on a 12 hour dial, with some 12 hour watches (see Movado) not including any markings at all. Because of this, a 24 hour dial will be confusing and require more thought at first than a 12 hour dial.
Aficionados of 24 hour watches point out that this layout is more natural, with the dial representing a full day and the hour hand proceeding through a fixed motion. They find that they can more easily "feel" the progress of the day, with each half of the dial representing day or night. They also point out that a 24 hour dial eliminates any confusion over AM and PM, which is especially valuable indoors or when traveling.
24 hour subdial
Many watches also feature a 24 hour subdial, often for a second timezone but sometimes these subdials even show the same time as the main dial. These are not "24 hour watches" per se, but they do have 24 hour features.
For example, the Jaeger-LeCoultre Reverso Duoface pictured at left displays the same time on the larger main dial and smaller 24 hour dial. Note that the smaller dial has the 24 oriented to the left with night and day hemispheres arranged left and right. This face even includes a label for night and day to show this intent. Many similar subdials feature a dark band from 6 PM to 6 AM for the same reason.
Note that many GMT watches, including the famous Rolex GMT-Master II, include a second centrical hour hand using an outer 24 hour dial. These can be configured for any timezone, not just GMT/UTC, and some people will set them to the current local time. This makes a GMT watch both a 12- and 24-hour watch.
Many of these watches with a secondary 24 hour dial can help users become accustomed to the daily cycle inherent in this display. But the non-standard orientation of these dials interferes with this goal.