Rectangles too, any four corners played at once. It's because the sensors are horizontal and vertical stripes of velostat, so every individual stripe can only sense pressure, the location is gather by combining the information of the overlapping stripes.

from their webpage [1]:

> If 3 note pads are pressed that are 3 corners of a rectangle, presses to a note pad that is the 4th corner of that rectangle will be ignored.

[1] http://www.rogerlinndesign.com/ls-specs.html