The triangular membership function has a singular apex. The apex can be located at any position within (and including) the endpoints. Note that the apex is at the 100% set membership region, and the enpoints are at 0%. What this does is  to classify data that falls within the endpoints as of some relative percent of set membership. Clicking the  triangle picture (beneath the triangle option button) will cause the picture of the triangle to change shape: A left extreme apex, a right extreme apex, and an apex that floats somewhere in between. Entering in the endpoint and apex data will specify the position along the graph that this function will cover. Note that the units used are relative to the graph.
