The Germans increased the difficulty of decrypting Enigma messages by decreasing the interval between changes in the order of the rotors from quarterly, initially, to monthly in February 1936, then daily in October of that year, when they also increased the number of plugboard leads from six to a number that varied between five and eight. This made the Biuro's grill method much less easy, as it relied on "unsteckered" letter pairs. The German navy was more security-conscious than the army and air force, and in May 1937 it introduced a new, much more secure, indicator procedure that remained unbroken for several years.