In reality, you are actually coinciding given two points with the points of your ruler. Then you will say, these points are separated by x unit in length.
Relax. Until recently, lurking in the dark recesses of mathematical existence, there might have been a really weird sphere of 254 dimensions, or 510, or 1,026. In fact, for all you knew, you might have had to worry about weird spheres when visiting any space with numbers of dimensions of the type 2k - 2.