The ToolTip class works on the control level. So there's no way to apply a tooltip to part of a control. Typically, what you would do in a case like this is use the MouseMove event to change the tooltip on the control based on which button the mouse is over.

The tricky part is determining which button the mouse is over. But this is easy once you know how. 

        private void ultraTextEditor1_MouseMove(object sender, MouseEventArgs e)
        {
            UltraTextEditor ultraTextEditor = (UltraTextEditor)sender;
            UIElement element = ultraTextEditor.UIElement.LastElementEntered;
            if (element != null)
            {
                EditorButtonBase editorButton = element.GetContext(typeof(EditorButtonBase)) as EditorButtonBase;
                if (editorButton != null)
                {
                    Debug.WriteLine(editorButton.Key);
                    return;
                }
            }

            Debug.WriteLine("No editor button found");
        }