Documentation on guarded variables.

doc-src/content/stylesheets/main.sass

@@ -90,6 +90,8 @@ body.reference
     width: 100px
     height: 20px
     +border-radius(3px)
+  a.help
+    font-size: 75%
 
   
   .tipsy

doc-src/content/tutorials/configurable-variables.haml

@@ -0,0 +1,30 @@
+---
+title: Working with Configurable Variables
+classnames:
+  - tutorial
+---
+
+:markdown
+  Working with Configurable Variables
+  ===================================
+
+  There are two ways of defining a variable in Sass. The first, most common, approach is
+  simple assignment. For example:
+
+      !my_constant = #fedcba
+
+  The second approach is called guarded assignment. In this case, the constant is only
+  set if it does not already have a value. For example:
+  
+      !my_constant ||= #fedcba
+  
+  Many compass modules use guarded assignment to allow you to set defaults for that module.
+  In order for these configurable variables to work correctly, you must set the variables
+  *before* you import the module. For example:
+  
+      !blueprint_grid_columns = 12
+      @import blueprint/grid
+  
+  Because of this, it is common to have one or more partials
+  that set the constants first and get imported before any other imports in your stylesheet(s).
+  This is commonly referred to as the "base" stylesheet and is usually named `_base.sass`.

doc-src/layouts/partials/sidebar.haml

@@ -11,6 +11,9 @@
   %li
     %a{:href => "/docs/best_practices/"}
       Best practices
+    %ul
+      %li
+        %a{:href => "/docs/tutorials/configurable-variables/"} Configurable Variables
   %li
     %a{:href => "/docs/frameworks/"}
       Working with plugins and frameworks