1. A touch-sensor comprising: a substrate; a plurality of sensor electrodes disposed on the substrate, the plurality of sensor electrodes configured to sense an object proximate to the plurality of sensor electrodes; and a processor coupled to the plurality of sensor electrodes, the processor configured to: detect a first presence of an object proximate to the touch-sensor; detect a second presence of an object proximate to the touch-sensor, the second presence starting after, and overlapping in time with, the first presence; and initiate a gesture signal indicating an occurrence of a multiple-presence gesture responsive to a set of criteria being met, the set of criteria comprising detecting first and second presences of objects, wherein the second presence starts after, and overlaps in time with, the first presence; from a first time associated with a first Z value on one side of a first threshold until a second time later than the first time associated with a second Z value on an opposite side of the first threshold, wherein all Z values associated with times between the first time and second time are all past a reference Z value, provide positional information representing motion of the object on the touch-sensor; and after the second time, initiate a gesture signal indicating the occurrence of the gesture.