In numerical modelling and general mathematics, a variable whose value changes as a function of another variable, i.e. the latter is first specified and the latter then calculated. The specified variables are called independent variables. Examples of variables that are usually dependent in numerical modelling include velocities, temperatures, and densities, with the independent variables usually being the spatial positions and time, although some variables can be either depending on the situation. For example, when pressure coordinates are used the pressure is an independent variable and the height or depth a dependent one, but when level coordinates are used the positions are reversed.