Subject of Bach's <a href="https://en.wikipedia.org/wiki/The_Art_of_Fugue" class="extiw" title="en:The Art of Fugue">The Art of Fugue</a>.
<a href="//commons.wikimedia.org/wiki/User:Wereon" title="User:Wereon">Wereon</a>
<span class="int-own-work" lang="en">Own work</span>