title_body: What is the key to realistic inventories Shy of simply recreating or stitching together natural languages' phonological inventories, what is the solution for making a naturalistic conlangs' inventory naturalistic?
upvoted_answer: You should think of phonology  in terms of distinction. You have to distinguish certain consonants and vowels from others, and you have to figure out the best way to do that.
  
  A realistic inventory has spread out places of articulation, often symmetrical. It's much easier to make a distinction between spread-out consonants that between consonants that are articulated similarly. That's why this is a really common inventory:
  
               Front Central Back
  Close        i             u
  Near-Close
  Mid-Close    
  Mid          e             o
  Mid-Open
  Near-Open
  Mid-Open           a
  
  
  And this is unheard of:
  
               Front Central Back
  Close        i             u
  Near-Close   ɪ             ʊ
  Mid-Close          ɘ
  Mid                   
  Mid-Open
  Near-Open
  Mid-Open    
  
  
  You should always make sure a language makes distinctions that are spread apart, e.g. /p t k/, rather than distinguishing three types of t-sounds /t̼ t ʈ/ without also having a distinction from /p k/ (unless for some reason you're making a language to be spoken by aliens without lips, for instance). There's also no reason to have only three consonants /p d kʰ/; you should either have only one set (such as /p t k/), or have a set for each of them (i.e. /p pʰ b, t tʰ d, k kʰ g/).
  
  Every rule has its exceptions, but this is true of most natural languages, and you should use this as a starting point.
  
  (Some more details can be found in my answer on Linguistics, from which I copied some of the examples)