title: How to illustrate multiple threads in sequence diagram?
body: <p>How can you clearly illustrate multiple threads of execution in a sequence diagram or similar diagram?  </p>
  
  <p>I haven't been able to find any clear examples.  All diagrams I see are used to illustrate a single thread.</p>
  
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  <p><strong>Update</strong>: The accepted answer was the best example I saw but it does leave a fair bit to be desired.  I ended up illustrating the threads in separate sequence diagrams.  I'm not sure if the sequence diagram necessarily works for multiple threads.</p>