Three-dimensional Lexis diagram with two life lines.

<p>Abscissa, ordinate and applicate (z-axis) represent calendar time , age and duration , respectively. The life lines start at birth and end at death The first subject (blue line segments) contracts the disease at . Then, the life line changes its direction. The second subject (red line segment) does not contract the disease, the life line remains in the <i>t</i>-<i>a</i>-plane.