Illustration showing the measuring of the <a href="https://en.wikipedia.org/wiki/Angle" class="extiw" title="en:Angle">angle</a> <var>θ</var>.
<a href="//commons.wikimedia.org/wiki/User:Gustavb" title="User:Gustavb">Gustavb</a>
CC BY-SA 3.0
<span class="int-own-work" lang="en">Own work</span>