After you have selected the FIRST corner of a Set Window with the Add Set Window option, you will notice that if the cursor moves beyond the stereonet perimeter, it will "wrap around" and re-appear on the opposite side of the stereonet, with the window still attached.
This allows data near the perimeter, on opposite sides of the stereonet, to be selected as one Set, as illustrated below.
Wrapped Set Window
Mean Vector Calculation for Wrapped Set Windows
The mean vector calculation for Sets created with a regular (i.e. non-wrapping) Set Window, is simply vector addition of all of the poles within a window, which is then normalized to the sphere boundaries.
When pole vectors are clustered near the equator, and plot on opposite sides of the stereonet, A MEAN ORIENTATION CALCULATED FROM THE LOWER HEMISPHERE ALONE WILL BE INCORRECT!! The wrapping Set window capability of Dips automatically accounts for this situation. The poles within a wrapped Set window that plot on the opposite side of the stereonet, are incorporated into the vector addition AS NEGATIVE poles (i.e. plunge = – plunge , trend = trend + 180), so that the mean will be correctly calculated.
NOTE: if you are using MULTIPLE WRAPPED WINDOWS FOR A SINGLE SET, all windows must "wrap" in the same direction, so that the mean is correctly calculated.