The word geometry literally means earth measurement. When geometry was developed by the ancients of various cultures, it was probably for earth measurement, that is, surveying. In mathematics today, geometry generally refers to the study of curves and surfaces. Different branches of geometry—differential geometry, algebraic geometry, geometric analysis—are distinguished in part by the objects of study and in part by the different sets of tools brought to bear upon the objects of study. In differential geometry, for example, the objects are curves and surfaces in complex space and the tools arise largely from differential calculus. Algebraic geometry is the study of objects that can be described using rational functions and the tools arise typically, but not always, from modern algebra.