<p>&#39;xlogo&#39; is close to the simplest possible X11 program. But it has so many uses.</p><p>• A ruler! Make an xlogo, drag it to a size, and if your WM shows a size tooltip as you drag, you can measure distances in pixels. Any X11 app works for this (unless it&#39;s sized in character cells or something) but you want one that doesn&#39;t try to do any actual _stuff_ as a side effect.</p><p>• Also, in reverse, &#39;xlogo -geometry 1920x1080&#39; and then you have a nice rectangle you can check if some other window fits into.</p><p>• An alarm clock! Sleep until a certain time and then open an xlogo too large to miss. Doesn&#39;t disturb your colleagues, but if you&#39;re looking away when it appears, that&#39;s fine, it&#39;s still there.</p><p>• A colour chart! What do those names like &#39;OliveDrab&#39; mean anyway? &#39;xlogo -bg thingy&#39;</p><p>• An inert X session-lifetime process, if anyone still uses those.</p><p>• Just occasionally, you might even want to display the X11 logo, or find out how to. (The xlogo source is the official definition of the logo.)</p>