title: What's the difference between /dev/tty.* and /dev/cu.* on macOS?
body: <p>Each serial device shows up twice in <code>/dev</code>, once as a <code>tty.*</code> and once as a <code>cu.*</code>.</p>
  <p>What is the <code>cu.*</code> device?  How does it differ from the <code>tty.*</code> device?</p>
  <pre><code>ls -l /dev/*.usbmodem621
  </code></pre>
  <p>Output:</p>
  <pre><code>crw-rw-rw-  1 root  wheel   11,   5 Dec 25 18:00 /dev/cu.usbmodem621
  crw-rw-rw-  1 root  wheel   11,   4 Dec 25 18:00 /dev/tty.usbmodem621
  </code></pre>