A sector dial is a watch dial that includes simple markings dividing it into sub-sections.
A traditional sector dial includes concentric circles inside and outside the hour or minute markings with solid lines linking these circles at each. An hour sector would divide the space between these circles into 12 sections or sectors, while a minute sector has 60 sections, often with stronger markings on the hour. The look of a minute sector dial gives rise to the "railroad" nickname, though they should not be confused with a railroad watch.
Sector dials were popular in trench watches in World War I, as they could be placed on a table and used to show which hour sector a direction corresponded to. Many officer's watches also have sector dials, as do marine watches, for the same reason.
Sector dials were used by Omega, Longines, and others, but it is the Patek Philippe sector dials that are most sought-after. The presence of this dial style can multiply the value of a Patek Philippe watch, sometimes even by an order of magnitude. Although officer's watches often have a sector dial, the Patek Philippe Officer's Watch family typically does not.
One common dial is the crosshair dial, as seen on Rolex Oyster Perpetual, Jaeger-LeCoultre Geophysic, and Omega Constellation and Omega Seamaster watches. In these watches, a "crosshair" marking centered on the dial extends from 12 to 6 and 9 to 3. Sometimes this crosshair extends all the way to the edge of the dial, but it often ends inside the hour markers.
Although many watch enthusiasts do not consider a crosshair dial to be a sector dial, the terms are commonly interchanged. Sometimes, both crosshair and sector dials are combined, as on some Patek Philippe Calatrava 96 models.