title: Using atan2 to find angle between two vectors
body: <p>I understand that:</p>
  
  <p><code>atan2(vector.y, vector.x)</code> = the angle between the <strong>vector and the X axis</strong>.</p>
  
  <p>But I wanted to know how to get the angle between <strong>two vectors</strong> using atan2. So I came across this solution:</p>
  
  <pre><code>atan2(vector1.y - vector2.y, vector1.x - vector2.x)
  </code></pre>
  
  <p>My question is very simple:</p>
  
  <p>Will the two following formulas produce the same number?</p>
  
  <ul>
  <li><p><code>atan2(vector1.y - vector2.y, vector1.x - vector2.x)</code></p></li>
  <li><p><code>atan2(vector2.y - vector1.y, vector2.x - vector1.x)</code></p></li>
  </ul>
  
  <p>If not: <strong>How do I know what vector comes first in the subtractions?</strong></p>