Criteria for the straightness of a curve

The following three criteria for the straightness of the a are proved:
1. A curve in an affine $n$-dimensional space is rectilinear if and only if each of its chords has a common point with the arc contracted by it, which is different from their common ends.
2. A curve in a Euclidean $3$-dimensional space is rectilinear if and only if any two of its oriented arcs are similar.
3. A rectifiable curve in a Euclidean $n$-dimensional space is rectilinear if and only if any two of its oriented arcs are similar.