This contribution presents the first results of an experimental research aimed at finding an unified description of the first stages of a rock comminution process. With reference to the case of the jaw crushers and to the specific subject of the comminution law, the tests reviewed and analysed in this contribution have been carried out, at a laboratory scale, on five different materials (limestones, quartzite, serpentinite) at different reduction ratios and feed rates, and accurately recording both total power and idle running power. The analysis of the results has shown that: the grain size distribution of the products complies with a modified form of the Rosin-Rammler equation, already found to be well suited to representing the grain size distribution of blasted rock in previous works by the authors; observed trends of the consumed power at increasing reduction ratios is steeper than predicted by the classical Rittinger, Kick and Bond laws, Rittinger law representing however a closer approximation of the experimental results; and a more accurate description of the experimental results is provided by a modified form of the Rittinger law. Accuracy of the semi-empirical expression has been found to be usually better than ten per cent by comparing actual results with expected Results.