diff --git a/stack.go b/stack.go
index 46b8878..4a6a033 100644
--- a/stack.go
+++ b/stack.go
@@ -13,12 +13,18 @@ const (
 	Vertical
 )
 
-// A Stack stacks controls horizontally or vertically within the Stack's parent, alotting each the same size.
+// A Stack stacks controls horizontally or vertically within the Stack's parent.
+// A horizontal Stack gives all controls the same height and their preferred widths.
+// A vertical Stack gives all controls the same width and their preferred heights.
+// Some controls may be marked as "stretchy": when the Window they are in changes size, stretchy controls resize to take up the remaining space after non-stretchy controls are laid out. If multiple controls are marked stretchy, they are alloted equal distribution of the remaining space.
 type Stack struct {
 	lock			sync.Mutex
 	created		bool
 	orientation	Orientation
 	controls		[]Control
+	stretchy		[]bool
+	xpos, ypos	[]int		// caches to avoid reallocating these each time
+	width, height	[]int
 }
 
 // NewStack creates a new Stack with the specified orientation.
@@ -29,14 +35,27 @@ func NewStack(o Orientation, controls ...Control) *Stack {
 	return &Stack{
 		orientation:	o,
 		controls:		controls,
+		stretchy:		make([]bool, len(controls)),
+		xpos:		make([]int, len(controls)),
+		ypos:		make([]int, len(controls)),
+		width:		make([]int, len(controls)),
+		height:		make([]int, len(controls)),
 	}
 }
 
+// SetStretchy marks a control in a Stack as stretchy.
+func (s *Stack) SetStretchy(index int) {
+	s.lock.Lock()
+	defer s.lock.Unlock()
+
+	s[index] = true			// TODO explicitly check for index out of bounds?
+}
+
 func (s *Stack) make(window *sysData) error {
 	for i, c := range s.controls {
 		err := c.make(window)
 		if err != nil {
-			return fmt.Errorf("error adding control %d: %v", i, err)
+			return fmt.Errorf("error adding control %d to Stack: %v", i, err)
 		}
 	}
 	s.created = true