Small signs found. Two rectangular bars of equal width overlap, and the overlapping part must be a diamond. Tell me why.

The height of both sides of the diamond verified by vertex is equal, because the width of the paper strip is equal. ASA can easily prove that two red triangles are the same and a group of adjacent sides are equal. Because the upper and lower sides of the note are parallel, the two groups of opposite sides are parallel to form a parallelogram. So it is a diamond.