What for? Use the button to select a triangle congruence theorem: SSS, SAS, ASA, SAA, SSA. Then specify the given triangle sides and/or angles with either the sliders and/or input boxes. See the unspecified parts solved and displayed in the graphics view. If your specified values for SSA lead to the ambiguous case in which two triangles are possible, an "ambiguous" button will appear and allow you to display either one or both cases. The "labelselector" slider allows you to cycle through all possible combinations of labeling vertices/angles A, B, C and their opposite sides a, b, c respectively. Panning/Zooming On a touchscreen device, swiping and pinching allows panning and zooming. On a non-touchscreen device, click-dragging allows panning, and Ctrl + or Ctrl - allows zooming (Command + or Command - on Mac). Before trying keyboard shortcuts, deselect all objects by clicking in an empty part of the graphics view. A mouse scroll wheel may also allow zooming. Invalid triangles? If the triangle disappears, that means you've specified settings that do not create a real triangle. (Maybe that's obvious?) For some settings, the angle will appear external to the triangle. Such angles will exceed their respective slider values by 180°, and the sum of all displayed angles will exceed 180°. I'll leave it up to you to determine whether such triangles are valid! When faced with such a dubious triangle, switching between SSS, SAS, ASA, SAA, SSA may cause some sliders to disappear, but the input boxes will still be visible. Entering valid values into the input boxes will cause the sliders and triangle to re-appear.