diff --git a/doc/theme-format.txt b/doc/theme-format.txt
index bb79d4534..cefc31627 100644
--- a/doc/theme-format.txt
+++ b/doc/theme-format.txt
@@ -22,6 +22,16 @@ This document has separate sections for each format version. You may
 want to read the document in reverse order, since the base features
 are discussed under version 1.
 
+New Features in Theme Format Version 3.1
+========================================
+
+Additional predefined variables are added for positioning expressions:
+
+ frame_x_center: the X center of the entire frame, with respect to the
+     piece currently being drawn.
+ frame_y_center: the Y center of the entire frame, with respect to the
+     piece currently being drawn.
+
 New Features in Theme Format Version 3
 ======================================
 
diff --git a/src/ui/theme-parser.c b/src/ui/theme-parser.c
index 9dad99fbd..b2d8bf77d 100644
--- a/src/ui/theme-parser.c
+++ b/src/ui/theme-parser.c
@@ -38,7 +38,7 @@
  * look out for.
  */
 #define THEME_MAJOR_VERSION 3
-#define THEME_MINOR_VERSION 0
+#define THEME_MINOR_VERSION 1
 #define THEME_VERSION (1000 * THEME_MAJOR_VERSION + THEME_MINOR_VERSION)
 
 #define METACITY_THEME_FILENAME_FORMAT "metacity-theme-%d.xml"
diff --git a/src/ui/theme.c b/src/ui/theme.c
index 0bb43655a..367423371 100644
--- a/src/ui/theme.c
+++ b/src/ui/theme.c
@@ -2188,6 +2188,10 @@ pos_eval_get_variable (PosToken                  *t,
         *result = env->title_width;
       else if (t->d.v.name_quark == env->theme->quark_title_height)
         *result = env->title_height;
+      else if (t->d.v.name_quark == env->theme->quark_frame_x_center)
+        *result = env->frame_x_center;
+      else if (t->d.v.name_quark == env->theme->quark_frame_y_center)
+        *result = env->frame_y_center;
       else
         {
           g_set_error (err, META_THEME_ERROR,
@@ -2229,6 +2233,10 @@ pos_eval_get_variable (PosToken                  *t,
         *result = env->title_width;
       else if (strcmp (t->d.v.name, "title_height") == 0)
         *result = env->title_height;
+      else if (strcmp (t->d.v.name, "frame_x_center") == 0)
+        *result = env->frame_x_center;
+      else if (strcmp (t->d.v.name, "frame_y_center") == 0)
+        *result = env->frame_y_center;
       else
         {
           g_set_error (err, META_THEME_ERROR,
@@ -3437,6 +3445,8 @@ fill_env (MetaPositionExprEnv *env,
       env->right_width = info->fgeom->right_width;
       env->top_height = info->fgeom->top_height;
       env->bottom_height = info->fgeom->bottom_height;
+      env->frame_x_center = info->fgeom->width / 2 - logical_region.x;
+      env->frame_y_center = info->fgeom->height / 2 - logical_region.y;
     }
   else
     {
@@ -3444,6 +3454,8 @@ fill_env (MetaPositionExprEnv *env,
       env->right_width = 0;
       env->top_height = 0;
       env->bottom_height = 0;
+      env->frame_x_center = 0;
+      env->frame_y_center = 0;
     }
   
   env->mini_icon_width = info->mini_icon ? gdk_pixbuf_get_width (info->mini_icon) : 0;
@@ -4845,6 +4857,8 @@ meta_theme_new (void)
   theme->quark_icon_height = g_quark_from_static_string ("icon_height");
   theme->quark_title_width = g_quark_from_static_string ("title_width");
   theme->quark_title_height = g_quark_from_static_string ("title_height");
+  theme->quark_frame_x_center = g_quark_from_static_string ("frame_x_center");
+  theme->quark_frame_y_center = g_quark_from_static_string ("frame_y_center");
   return theme;
 }
 
diff --git a/src/ui/theme.h b/src/ui/theme.h
index ddf777d4d..02a61566a 100644
--- a/src/ui/theme.h
+++ b/src/ui/theme.h
@@ -819,6 +819,8 @@ struct _MetaTheme
   GQuark quark_icon_height;
   GQuark quark_title_width;
   GQuark quark_title_height;
+  GQuark quark_frame_x_center;
+  GQuark quark_frame_y_center;
 };
 
 struct _MetaPositionExprEnv
@@ -834,6 +836,8 @@ struct _MetaPositionExprEnv
   int bottom_height;
   int title_width;
   int title_height;
+  int frame_x_center;
+  int frame_y_center;
   int mini_icon_width;
   int mini_icon_height;
   int icon_width;