Before receiving the French intelligence material, Rejewski had made a careful study of Enigma messages, particularly of the first six letters of messages intercepted on a single day. For security, each message was encrypted using different starting positions of the rotors, as selected by the operator. This message setting was three letters long. To convey it to the receiving operator, the sending operator began the message by sending the message setting in a disguised form—a six-letter indicator. The indicator was formed using the Enigma with its rotors set to a common global setting for that day, termed the "ground setting", which was shared by all operators. The particular way that the indicator was constructed introduced a weakness into the cipher.