三等分角线（Trisectrix）是可以用来三等分任意角的曲线。若只用标准的尺规作图，不配合曲线或是有刻度的直尺，三等分角是不可能的。有许多的曲线可以作为三等分角的辅助，而进行三等分角的方式也各有不同。以下是一些三等分角线：

另一个相关曲线是等分角线（sectrix），是可以将任意角分为整数个的曲线。以下是一些等分角线：

In geometry, a trisectrix is a curve which can be used to trisect an arbitrary angle. Such a method falls outside those allowed by compass and straightedge constructions, so they do not contradict the well known theorem which states that an arbitrary angle cannot be trisected with that type of construction. There are a variety of such curves and the methods used to construct an angle trisector differ according to the curve. Examples include: