Histograms of the absolute value of the difference between the PA of the maser PM, PM PA, and the PA of the maser position vector from the YSO, POS PA (upper panel), and of the difference between the PM PA and the PA of the preferential direction of the water maser PMs, PD_PM (lower panel). To produce these histograms we have selected the POETS targets belonging to the subgroup N-UNC (see Table A.1) with at least two measured PMs. The histogram bin is 20° and the histogram values are normalized by the total number of considered PMs. The source-average error on the difference PM PA − POS PA and corresponding standard deviation are 32° and 6°. The source-average error on the PM PA and corresponding standard deviation are 9° and 3°.