回形针，又称回纹针、万字夹、曲别针、纸夹，是一种能够暂时性地固定纸张的文具，其形状不一，均具有双循环结构，和 订书机、长尾夹的功能类似。回形针一般由不锈钢、白铁、铜、塑胶制造，材料的特性为可弯曲、不易折断。

没有回纹针的年代，只能在纸上打洞。挪威专利局职员约翰‧瓦莱（Johann Vaaler）一度被认为是回纹针发明人。约翰‧瓦莱开发出第一支三角形回纹针，1899年在德国申请专利（当时挪威没有专利法），1901年取得美国专利，专利号：675761，但较少实用性，因为它缺乏转最后一个弯的电线。他的版本从来没有制造和销售。二次大战期间由挪威国王带头「在衣服上佩戴回纹」以作为爱国心的象征，用于挑衅侵略者德国，挪威的字典会提到 Vaaler是回纹针发明人。1990年2月，挪威首都奥斯陆郊外，有一座高达5米的回纹针形的不锈钢纪念碑。

A paper clip (or sometimes paperclip) is an instrument used to hold sheets of paper together, usually made of steel wire bent to a looped shape. Most paper clips are variations of the Gem type introduced in the 1890s or earlier, characterized by the almost two full loops made by the wire. Common to paper clips proper is their utilization of torsion and elasticity in the wire, and friction between wire and paper. When a moderate number of sheets are inserted between the two "tongues" of the clip, the tongues will be forced apart and cause torsion in the bend of the wire to grip the sheets together. Too many sheets will cause the elastic limit of the material to be exceeded, resulting in permanent deformation.