diff --git a/object.go b/object.go
index 078a425..723dec4 100644
--- a/object.go
+++ b/object.go
@@ -98,10 +98,13 @@ type Align int; const (
 	AlignEven               // similar to justified text
 )
 
+// Object is any obscreen object. Each object must be linked to a box, even if
+// it is that box.
 type Object interface {
 	Box () Box
 }
 
+// Box is a basic styled box.
 type Box interface {
 	Object
 
@@ -136,28 +139,38 @@ type Box interface {
 	OnScroll      (func (deltaX, deltaY float64)) event.Cookie
 	OnKeyDown     (func (key input.Key, numberPad bool)) event.Cookie
 	OnKeyUp       (func (key input.Key, numberPad bool)) event.Cookie
-
-	ContentBounds         () image.Rectangle
-	ScrollTo              (image.Point)
-	OnContentBoundsChange (func ()) event.Cookie
-}
-
-type TextBox interface {
-	Box
-	SetTextColor (color.Color)
-	SetFace      (font.Face)
-	SetHAlign    (Align)
-	SetVAlign    (Align)
 }
 
+// CanvasBox is a box that can be drawn to.
 type CanvasBox interface {
 	Box
 	SetDrawer  (canvas.Drawer)
 	Invalidate ()
 }
 
-type ContainerBox interface {
+// ContentBox is an abstract box that has some kind of content. Its only purpose
+// is to be embedded into TextBox and ContainerBox.
+type ContentBox interface {
 	Box
+	SetOverflow           (horizontal, vertical bool)
+	ContentBounds         () image.Rectangle
+	ScrollTo              (image.Point)
+	OnContentBoundsChange (func ()) event.Cookie
+}
+
+// TextBox is a box that contains text content.
+type TextBox interface {
+	ContentBox
+	SetTextColor (color.Color)
+	SetFace      (font.Face)
+	SetHAlign    (Align)
+	SetVAlign    (Align)
+}
+
+// ContentBox is a box that can contain child objects. It arranges them
+// according to a layout rule.
+type ContainerBox interface {
+	ContentBox
 	SetGap    (Gap)
 	Add       (Object)
 	Delete    (Object)
@@ -168,6 +181,12 @@ type ContainerBox interface {
 	SetLayout (Layout)
 }
 
+// Layout can be given to a ContainerBox to arrange child objects.
+type Layout interface {
+	Arrange (image.Rectangle, Gap, []Box)
+}
+
+// Window is an operating system window. It can contain one object.
 type Window interface {
 	SetRoot  (Object)
 	SetTitle (string)
@@ -183,11 +202,8 @@ type Window interface {
 	OnClose  (func ()) event.Cookie
 }
 
+// MainWindow is a top-level operating system window.
 type MainWindow interface {
 	Window
 	NewChild (image.Rectangle) (Window, error)
 }
-
-type Layout interface {
-	Arrange (image.Rectangle, Gap, []Box)
-}