If you've ever coded for multiple display types, especially the web and then iPhone screens, and  just  for grins throw in some Android screens including a few "Tablets" just for fun. My experience is you can pretty much just lump the Android screens in with your PC or MAC screens.  They tend to be a fairly common size and they are fairly well behaved. But when it comes to iOS screens, all bets are off.   Is it a small phone, or a big iPad., Is it HD display or not.   What is the input method, touch, type, or Pen?  Each of these require a different level of precision.  As I am writing the post today, we already have well over 10 pixel densities.  Common ones include  ldpi mdpi hdpi xhdp xxhdpi xxxhdpi nodpi tvdpi To help with all of this, I found two great sources.  One does a  great job of explaining iOS displays. Nowadays, not just iOS displays but may Android displays as well.  The other does a great job of everything else. I checked out my little Pulse Oximeter devic