The second kind of atoms are made up from signs only. Sometimes it may be necessary to have an atom beginning with a capital letter or a digit. If an atom is enclosed in single quotes, then the atom may have any characters in its name. The special underscore character may be inserted in the middle of an atom to improve legibility. The second kind of term used in Prolog is the variable.
Variables look like atoms, except they begin with a capital letter or an underscore sign.
A variable should be thought of as standing for some object that we may not be able to name. The third kind of term is the structure. A structure is a single object that consists of a collection of other objects, called components. The components are grouped together into a single structure for convenience in handling them. A structure is written in Prolog by specifying its functor, and its components.