Specifying a Ruby Version Like gems, developers can setup a dependency on Ruby. This makes your app fail faster in case you depend on specific features in a Ruby VM. This way, the Ruby VM on your deployment server will match your local one. You can do this by using the ruby directive in the Gemfile: ruby 'RUBY_VERSION', :engine => 'ENGINE', :engine_version => 'ENGINE_VERSION', :patchlevel => 'RUBY