Two mirror lines, m one and m two, cross at a point O with an angle φ between them. A triangle is reflected first across m one, giving a flipped copy, and then that copy is reflected across m two. The end result is the original triangle rotated about O by twice the angle between the mirrors — a rotation of 2φ. You can drag the triangle's corners, and rotate each mirror line. The angle φ between the mirrors and the resulting rotation angle 2φ are shown live, and the doubly reflected triangle always matches a pure rotation of the original about O. An info button opens a drawer with the explanation.