Parallel lines are denoted by the parallel symbol placed betwen the notation for two lines. For example, the expression means that the line is parallel with the line which corresponds to the image shown below.

As shown in the image, when labeling a diagram, sometimes authors also include arrows along the lengths of the lines that are parallel two each other as another way to express the relationship of the lines to each other.

A line is written as two letter with a line overtop, where the letters represent the end points of the line.

The symbol for two perpendicular lines is a horizontal line with another line drawn perpendicular to it.