title: What is the unit of measurement in Xcode
body: <p>This might sound silly but when i'm working with sizes in Xcode, the total frame size is equal to 320 * 568 ( width * height ). I'm fine with that. But the resolution of the phone is actually different. The iphone 4S is 640*960 &amp; the 5 and next generations are 640*1136.</p>
  
  <p>I know the last one is exactly double of what xcode is using as units, so my question is : </p>
  
  <blockquote>
    <p>What unit of measurement does XCode use? If it pixels, why not use the
    phone size/resolution as reference?</p>
  </blockquote>