A transversal line can be a straight or curved line that intersects another line. For example, if one line intersects two parallel lines, then it is a transversal line. When a transversal line does this, it forms a point of intersection and four angles. The four angles arise in the areas between the two lines. For example, if a line intersects another line to form a "+" shape, then there are four angles that form around the point of intersection.
When a transversal line intersects another line, the four angles that form are numbered. These angles are generally numbered clockwise, with the angle in the top left corner being number 1 and the number in the top right corner being 2. In many cases, these numbers may also be given variables, which act as symbols that represent the degree of the angle. In geometry, you usually find the degree of the angle by using clues from other angles of intersection.
One of the basic fundamentals of geometry is that, when a transversal intersects a line, the resulting angles that are opposite of one another are equal. In other words, when four angles are formed by a transversal intersecting a line, the angles opposite of one another have the same degree of measurement. This theorem helps find the area of triangles, trapezoids and other complex shapes that are formed by transversals.
When two parallel lines are bisected by a transversal, the angles surrounding one point of intersection are the same as that of the other intersection. For example, when a transversal intersects two parallel lines, the pairs of opposite angles for one intersection might be 30 and 150 degrees. In the case of parallel lines, the angles in the other parallel line will be the same as the first, and so we know that the second parallel line also has opposite pairs of angles that measure 30 and 150 degrees, respectively.