diff --git a/collects/drscheme/private/main.ss b/collects/drscheme/private/main.ss
index 48fedc6e4c..9a92dbdefa 100644
--- a/collects/drscheme/private/main.ss
+++ b/collects/drscheme/private/main.ss
@@ -57,6 +57,7 @@
                                      (finder:default-filters)))
       (application:current-app-name (string-constant drscheme))
 
+  (preferences:set-default 'drscheme:defs/ints-horizontal #f boolean?)
   (preferences:set-default 'drscheme:unit-window-max? #f boolean?)
   (preferences:set-default 'drscheme:frame:initial-position #f 
                            (λ (x) (or (not x)
@@ -215,7 +216,9 @@
                            (string-constant switch-to-module-language-automatically)
                            editor-panel)
 
-           
+           (make-check-box 'drscheme:defs/ints-horizontal
+                           (string-constant interactions-beside-definitions)
+                           editor-panel)
            ;; come back to this one.
            #;
            (letrec ([hp (new horizontal-panel% 
diff --git a/collects/drscheme/private/unit.ss b/collects/drscheme/private/unit.ss
index ac00f6057c..3ae33d4191 100644
--- a/collects/drscheme/private/unit.ss
+++ b/collects/drscheme/private/unit.ss
@@ -958,20 +958,23 @@ module browser threading seems wrong.
   ;   ;     ;;;;; ;   ;   ;    ;;;; 
                                     
                                     
-                                    
-      (define vertical-dragable/def-int%
-        (class panel:vertical-dragable%
-          (init-field unit-frame)
-          (inherit get-percentages)
-          (define/augment (after-percentage-change)
-            (let ([percentages (get-percentages)])
-              (when (and (= 1
-                            (length (send unit-frame get-definitions-canvases))
-                            (length (send unit-frame get-interactions-canvases)))
-                         (= 2 (length percentages)))
-                (preferences:set 'drscheme:unit-window-size-percentage (car percentages))))
-            (inner (void) after-percentage-change))
-          (super-new)))
+    
+    (define dragable/def-int-mixin
+      (mixin (panel:dragable<%>) ()
+        (init-field unit-frame)
+        (inherit get-percentages)
+        (define/augment (after-percentage-change)
+          (let ([percentages (get-percentages)])
+            (when (and (= 1
+                          (length (send unit-frame get-definitions-canvases))
+                          (length (send unit-frame get-interactions-canvases)))
+                       (= 2 (length percentages)))
+              (preferences:set 'drscheme:unit-window-size-percentage (car percentages))))
+          (inner (void) after-percentage-change))
+        (super-new)))
+    
+    (define vertical-dragable/def-int% (dragable/def-int-mixin panel:vertical-dragable%))
+    (define horizontal-dragable/def-int% (dragable/def-int-mixin panel:horizontal-dragable%))
       
     (define super-frame%
       (drscheme:frame:mixin
@@ -3170,9 +3173,11 @@ module browser threading seems wrong.
                                               (let ([sel (send tabs-panel get-selection)])
                                                 (when sel
                                                   (change-to-nth-tab sel)))))))
-          [define resizable-panel (new vertical-dragable/def-int%
-                                    (unit-frame this)
-                                    (parent panel-with-tabs))]
+          [define resizable-panel (new (if (preferences:get 'drscheme:defs/ints-horizontal)
+                                           horizontal-dragable/def-int%
+                                           vertical-dragable/def-int%)
+                                       (unit-frame this)
+                                       (parent panel-with-tabs))]
           
           [define definitions-canvas #f]
           (initialize-definitions-canvas)
diff --git a/collects/string-constants/english-string-constants.ss b/collects/string-constants/english-string-constants.ss
index a5135674c5..7d9fb1058f 100644
--- a/collects/string-constants/english-string-constants.ss
+++ b/collects/string-constants/english-string-constants.ss
@@ -417,6 +417,7 @@ please adhere to these guidelines:
  (open-files-in-tabs "Open files in separate tabs (not separate windows)")
  (show-interactions-on-execute "Automatically open interactions window when running a program")
   (switch-to-module-language-automatically "Automatically switch to the module language when opening a module")
+  (interactions-beside-definitions "Put the interactions window beside the definitions window") ;; in preferences, below the checkbox one line above this one
  (limit-interactions-size "Limit interactions size")
  (background-color "Background Color")
  (default-text-color "Default text") ;; used for configuring colors, but doesn't need the word "color"