For each "top" and "bottom" part, we introduced an adimensional number referred to as "polarization index", PI = (ALd(Pole)-ALd(Body))/ALd(Tot), where ALd(Pole), ALd(Body) and ALd(Tot) were the projected area occupied by the Ld phase in the pole, body and entire half of the vesicle, respectively (Figure , top left). This allowed us to systematically categorize the situation of each "top" and "bottom" part of the confined vesicles ranging from a strong polarization of Lo domain (PI → -1), to a weak or the absence of polarization (PI ≈ 0) to a strong polarization in Ld domain (PI → +1). Figure , Figure and Table show the resulting polarization diagram where PI values for a large number of GUVs confined in straight channels with w = h = 30 µm were plotted in color scale for both the "top" (triangles pointing up) and the "bottom" (triangles pointing down) parts, as a function of the fraction of Ld phase and length of each confined GUV. First, this analysis confirmed the predominant role of the Lo/Ld phase ratio in determining the phase localization. When a strong mono-or bi-polarization occurred (high absolute value of PI), it was most of the time corresponding the phase in minority, in agreement with our previous observation. Interestingly, the strongest polarization effects were observed when the minority phase was not in a too small proportion. For instance, vesicles with two strongly polarized parts were mainly observed for Ld ≥ 20 mol% (Ld polarization) and Lo ≥ 40 mol% (Lo polarization). We can explain this effect by the fact that, when a phase is in a very small minority, the low number of initial domains reduced the probability that they fused into a single large domain. In that case, a small pole with the minority phase could be formed at the GUV tip but some minority phase domains also remained in the body region, decreasing de facto the PI absolute value. We also found that the aspect ratio of the confined GUV did not have a strong effect in the occurrence of polarization, except that highly elongated vesicles tended to have a lower PI absolute value. This was attributed to a fraction of the minority phase domains remaining in the elongated body region, due to a saturation of the free region and/or by kinetic trapping of the domains that were too far from the free curved region. Finally we found that, under conditions favoring polarization, GUVs were observed with two well-defined poles in many cases while a strict minimization of line tension would favor the formation of a single pole. We attribute the occurrence of such a bipolarization to the kinetic trapping of the minority phase domains when they entered in any of the two free curved regions imposed by the geometry. Confining GUVs in straight microchannels is thus a way to create polarized cell-mimicking compartments with partial yet efficient control over the nature and position of the pole(s).