There ain't no such thing as "a line"; we chose to invent markers (like a pencil) and straight edges
and using those we "draw something we choose to call a line" joining two Cartesian points.
The length of the line is really not what we measure by "ruler" (that's to give an approximation),
but is obtained by formula using the coordinates of two end points.
PROVE to me that your ruler is precise; it can't be, since the "lines" on the ruler have "width"...