title: What is <C-j> default action in insert mode in vim? Why I can't use it for YCM
body: <p>I found strange behavior about <kbd>Ctrl</kbd> + <kbd>J</kbd>. I called it strange because I can't get what action it does and. I tried some way to get what <kbd>Ctrl</kbd> + <kbd>J</kbd> really do with this:</p>
  
  <pre><code>:map &lt;C-j&gt;
  :imap &lt;C-j&gt;
  :vmap &lt;C-j&gt;
  :verbose map &lt;C-j&gt;
  :verbose imap &lt;C-j&gt;
  :verbose vmap &lt;C-j&gt;
  </code></pre>
  
  <p>they all give <code>no mapping found</code>.</p>
  
  <p>And then I tried this:</p>
  
  <pre><code>:unmap &lt;C-j&gt;
  </code></pre>
  
  <p>it gives me <code>E13: No such mapping</code></p>
  
  <p>I want to use it in YCM with:</p>
  
  <pre><code>`let g:ycm_key_list_select_completion = ['&lt;C-j&gt;']`
  `let g:ycm_key_list_previous_completion = ['&lt;C-k&gt;']`
  </code></pre>
  
  <p><code>&lt;C-k&gt;</code> does work but <code>&lt;C-j&gt;</code> doesn't. Instead, <code>&lt;C-j&gt;</code> (in insert mode) behave like <code>&lt;Enter&gt;</code>. So, what is <kbd>Ctrl</kbd> + <kbd>J</kbd> actually do in vim? And why I can't use it?</p>