A rhombus ABCD is shown — a quadrilateral with all four sides equal. Drag corner B to set the side length and one direction; drag corner D, whose distance from A is locked to the same side length; corner C follows so the shape stays a rhombus, and dragging A moves the whole figure. The two diagonals AC and BD are drawn and meet at the centre M. The theorem: in a rhombus the diagonals are perpendicular and bisect each other, so they cross at a right angle and AM equals MC and BM equals MD. Tick marks show the four equal sides, a right-angle mark sits at M, and the side length, diagonal lengths, and the angle between the diagonals are shown live. An info button opens a drawer with the explanation.