Once a language has been chosen, the next problem is to determine the primitive terms of the language. For plane Euclidean geometry, these are points and lines, although sometimes lines are defined as particular sets of points. Next the basic predicates have to be selected. There exist a number of different axiomatisations at the present moment. The most frequently used predicates are: the incidence relation (“a point \(a\) lies on a line \(A\)”), the betweenness relation (“point \(a\) lies between points \(b\) and \(c\)”), the equidistance relation (“the distance from point \(a\) to \(b\) is the same as the distance from point \(c\) to \(d\)”), the congruence relation (“a part of a line, determined by two point \(a\) and \(b\) is congruent to a part of a line, determined by two points \(c\) and \(d\)”). Note that it is not necessary that all of them occur in an axiomatisation. As an example, if lines are not introduced as primitive terms, then usually there is no incidence relation.