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Within the study of plane geometry, a transversal is a line that intersects two other lines at two separate points. Because each intersection of the transversal with a line creates four angles, in total the transversal produces eight angles. When a transversal intersects two lines that are parallel to one another, the eight angles that are produced have special relationships with one another. For this reason, the angles of a transversal are often studied as a means of determining whether two lines are parallel, because if they exhibit the same properties that they would if the lines were parallel, then it can be concluded that the lines are in fact parallel. When the lines crossed by the transversal are parallel, for example, several pairs of angles are created, some of which are congruent and others of which are supplementary, meaning that the sum of their values is equal to 180 degrees. The sum of all four angles created by the intersection of a transversal with a line is thus 360 degrees. Of the eight angles created by the transversal’s intersection of the two lines, four are called internal angles because they are found between the two lines that the transversal crosses, and the other four are called external, because they lie outside the area between the lines.