title: How does sbt choose which Scala version to use?
body: <p>I have multiple projects in my sbt build. I'm trying to upgrade to Scala 2.10 from 2.9.1, so in my build.sbt file I put  </p>
  
  <pre><code>scalaVersion := "2.10.0"
  </code></pre>
  
  <p>This seemed to work, because in my top-level project in sbt I get:  </p>
  
  <pre><code>&gt; scala-version
  [info] 2.10.0  
  </code></pre>
  
  <p>However, when I switch to one of the other projects:  </p>
  
  <pre><code>&gt; project web-client
  [info] Set current project to web-client (in build file:/C:/Users/...
  [web-client] $ scala-version
  [info] 2.9.1  
  </code></pre>
  
  <p>You see the version has now changed back to 2.9.1! How do I force the same Scala version to be used across all my projects?</p>