A half-twist joins one edge of a strip to the opposite edge. This identification creates a nonorientable surface: the two apparent sides belong to one continuous side.纸带扭转半周再首尾相接，一端的一侧便接到另一端的对侧。这种连接形成不可定向曲面，看似两面，实际是连续的同一面。
The gold point follows the boundary. After one turn it reaches the opposite edge of the original strip; after two turns it returns, revealing a single connected boundary.金色光点沿边界移动：绕一周到达原纸带的另一条边，再绕一周才回到起点，显出两条边已连成一条完整边界。