If you construct an arc through three points and then drag the three points so they are collinear, the second point determines the appearance of the arc. If the second point falls between the first and third points, the arc has zero angle measure but nonzero arc length; it is displayed as a segment that starts at the first selected point, passes through the second point, and ends at the third point. If the second point is collinear with, but not between, the first and third points, the arc is not well defined: it disappears and any measurements that depend on it are undefined.