Standard time is the synchronization of clocks within a geographical area or region to a single time standard, rather than using solar time or a locally-chosen meridian (longitude) to establish a local mean time standard. Historically, the concept was established during the 19th century to aid weather forecasting and train travel. Applied globally in the 20th century, the geographical areas became extended around evenly-spaced meridians into time zones which (usually) centered on them.