The terms "fortis" and "lenis" are variously regarded as having one single underlying phonetic correlate or many. An exploratory analysis of acoustic and aerodynamic data on contrasting stop series in a number of European and non-European languages confirms that a significant variation in peak intra-oral pressure and in articulatory stricture duration are two of the main factors differentiating these series. Two central questions are: (1) Is the contrast in peak pressure controlled by lung volume decrement or by the degree of glottal aperture? (2) Is the gesture for the "lenis" sound a truncated or a re-scaled version of the gesture for the "fortis" sound? A more detailed examination of the data from the non-European languages suggests that glottal aperture, rather than respiratory effort is the main physiological parameter underlying the pressure variation, and that each member of the opposition has a specific target peak pressure, rather than the "lenis" peak pressure being truncated by the early release of the articulatory closure.