Evolution of the orientation of the mean velocity gradient in the different datasets used to build the PV_rot diagrams and angular momentum distributions with respect to the PA of the velocity gradient observed at small scales Θ_small (PdBI C¹⁸O emission). The error bars of the orientation Θ are given in Table 2. They are smaller than 10° except for 7 of the 67 gradient measurements. For these 7 measurements, the large error bars are generally due to the absence of a clear gradient on either side of the central position of the source. Gradient measurements with large error are indicated by an empty circle. A typical error of ± 10° is shown on the first point of the plot.