Six simultaneous tuberculin skin tests on the same person (slightly retouched in 1 TU). The hands belong to a healthy Greek male 35, with known natural infection by Mycobacterium tuberculosis since 5 years (the first author). The six dots in the middle of erythemas represent the points of intradermal injection (Mantoux technique). The numbers between dots indicate the distance between injection points (in mm). The numbers under the heading 'TU' are the PPD-RT23 tuberculin units per injection (tuberculin dose). The numbers under the heading 'mm' are the induration diameters in mm. The small vertical lines |...| show the induration boundaries, and the arrows towards these lines, transversely to the log axis of the forearm, point to the ballpoint traces made in order to find boundaries with Sokal's method.