title: How is a high unicode codepoint expressed as two codepoints?
body: <p>I've <a href="https://stackoverflow.com/a/22115501/">seen</a> that >2 byte unicode codepoints like U+10000 can be written as a pair, like <code>\uD800\uDC00</code>. They seem to start with the nibble <code>d</code>, but that's all I've noticed.</p>
  
  <p>What is that splitting action called and how does it work?</p>