diff --git a/new/ui.h b/new/ui.h
index 105eb66..f9a0b26 100644
--- a/new/ui.h
+++ b/new/ui.h
@@ -32,13 +32,15 @@ struct uiSizing {
 	uiSizingSys *sys;
 };
 
+typedef strut uiContainer uiContainer;
+
 typedef struct uiControl uiControl;
 struct uiControl {
 	void *data;			// for use by implementations only
 	void *internal;			// for use by ui only
 	void (*destroy)(uiControl *);
 	uintptr_t (*handle)(uiControl *);
-	void (*setParent)(uiControl *, uintptr_t);
+	void (*setParent)(uiControl *, uiContainer *);
 	void (*removeParent)(uiControl *);
 	void (*preferredSize)(uiControl *, uiSizing *, intmax_t *, intmax_t *);
 	void (*resize)(uiControl *, intmax_t, intmax_t, intmax_t, intmax_t, uiSizing *);
@@ -54,7 +56,7 @@ struct uiControl {
 };
 void uiControlDestroy(uiControl *);
 uintptr_t uiControlHandle(uiControl *);
-void uiControlSetParent(uiControl *, uintptr_t);
+void uiControlSetParent(uiControl *, uiContainer *);
 void uiControlRemoveParent(uiControl *);
 void uiControlPreferredSize(uiControl *, uiSizing *, intmax_t *width, intmax_t *height);
 void uiControlResize(uiControl *, intmax_t, intmax_t, intmax_t, intmax_t, uiSizing *);
@@ -68,6 +70,42 @@ void uiControlDisable(uiControl *);
 void uiControlContainerEnable(uiControl *);
 void uiControlContainerDisable(uiControl *);
 
+// uiParent represents an OS control that hosts other OS controls.
+// It is used internally by package ui and by implementations.
+// uiWindow, uiTab, and uiGroup all use uiParents to store their controls.
+struct uiParent {
+	// Internal points to internal data.
+	// Do not access or alter this field.
+	void *Internal;
+
+	// Handle returns the window handle of the uiParent.
+	// On Windows, this is a HWND.
+	// On GTK+, this is a GtkContainer.
+	// On Mac OS X, this is a NSView.
+	uintptr_t (*Handle)(uiParent *p);
+#define uiParentHandle(p) ((*((p)->Handle))((p)))
+
+	// SetChild sets the uiControl that this uiParent relegates.
+	// It calls uiControl.SetParent().
+	void (*SetChild)(uiParent *p, uiControl *child);
+#define uiParentSetChild(p, child) ((*((p)->SetChild))((p), (child)))
+
+	// SetMargins sets the margins of the uiParent to the given margins.
+	// It then updates the uiParent to make the margins take effect.
+	// The units of the margins are backend-defined.
+	// The initial margins are all 0.
+	void (*SetMargins)(uiParent *p, intmax_t left, intmax_t top, intmax_t right, intmax_t bottom);
+#define uiParentSetMargins(p, left, top, right, bottom) ((*((p)->SetMargins))((p), (left), (top), (right), (bottom)))
+
+	// TODO Resize?
+
+	// Update tells the uiParent to re-layout its children immediately.
+	// It is called when a widget is shown or hidden or when a control is added or removed from a container such as uiStack.
+	void (*Update)(uiParent *p);
+#define uiParentUpdate(p) ((*((p)->Update))((p)))
+};
+uiParent *uiNewParent(uintptr_t);
+
 typedef struct uiWindow uiWindow;
 uiWindow *uiNewWindow(char *, int, int);
 void uiWindowDestroy(uiWindow *);