In geometry, once you learn how to do one construction, you can use that as one step in a larger construction. For instance, two parallel lines are both perpendicular to the same line. So, if you have a line AB, you can construct a new line DE that is parallel to AB as follows. First, construct a line CD that is perpendicular to the original line AB. Then construct a line DE that is perpendicular to line CD. Now line DE is parallel to line AB. No matter how long you draw lines AB and DE, they will never cross.