diff --git a/lib/youplot.rb b/lib/youplot.rb
index 320a687..f20d53f 100644
--- a/lib/youplot.rb
+++ b/lib/youplot.rb
@@ -6,6 +6,12 @@ require_relative 'youplot/parameters'
 require_relative 'youplot/command'
 
 module YouPlot
+  # @run_as_executable = true / false
+  # YouPlot behaves slightly differently when run as a command line tool
+  # and when run as a script (e.g. for testing). In the event of an error,
+  # when run as a command line tool, YouPlot will display a short error message
+  # and exit abnormally. When run as a script, it will just raise an error.
+  @run_as_executable = false
   class << self
     attr_accessor :run_as_executable
 
@@ -13,5 +19,4 @@ module YouPlot
       @run_as_executable
     end
   end
-  @run_as_executable = false
 end