Defining and Constructing Objects

Lines in the plane are represented by a line segment with arrows on both ends. We can identify two lines in the diagram: one that goes through points $R$ and $O,$ and one that goes through points $F$ and $L.$

We can use any two points on a line to name it. Point $R$ and $O$ are on the first line, so we can name it $\overleftrightarrow{RO}.$ The order of $R$ and $O$ doesn't matter and is, thus, an arbitrary choice. By the same principle, one name for the second line is $\overleftrightarrow{FL}.$