we call the length of the interval. In order that this may be uniquely determined it is necessary to fix arbitrarily the length of a definite interval; for example, we can put it equal to 1 (unit of length). Then the lengths of all other intervals may be determined. If we make the
linearly dependent upon a parameter
,
we obtain a line which has all the properties of the straight lines of the Euclidean geometry. In particular, it easily follows that by laying off
times the interval
