 Because the Vietnamese alphabet uses a complex system of diacritical marks, the keyboard needs 133 alphanumeric keys and a Shift key to cover all possible characters.Originally, VNI's input method utilized function keys (F1, F2, ...) to enter the tone marks, which later turned out to be problematic, as the operating system used those keys for other purposes.VNI then turned to the numerical keys along the top of the keyboard (as opposed to the numpad) for entering tone marks.