The general format for representing an event is: <[event modifier(s)-]...event type [-event detail]>. For key-binding Ctrl+Shift+Tab the format would be: 'Control-Shift-KeyPress-Tab'. In this case the event modifiers would be: Control- Shift-, the event type: Keypress, and the event detail: -Tab
The following code (in python 2.7.6) should make it clear:
from Tkinter import *
def key(event=None):
print 'You pressed Ctrl+Shift+Tab'
root = Tk()
frame = Frame(root, width=100, height=100)
frame.focus_set()
frame.bind('<Control-Shift-KeyPress-Tab>', key)
frame.pack()
root.mainloop()
EDIT: The above works well for Windows and Mac. For Linux, use
'<Control-ISO_Left_Tab>'.