Section 4 - 6Key Control & Record KeepingSpecial SymbolsGGM CTRA BAA BAAA1 AA2 AA3 A1 BA1 BA2 BA3 B1 GGM1 SKD1Change keys directly under higher level master keys take the name of their associated master key andadd numbers after the letter(s). SKD's, if used, are considered part of the system.CTR is the symbol for a control key. A top control key, capable of operating all the cores in a system, isshown in the schematic at the same level as the top master key.Cross keyingAlthough sometimes required, cross keying, either controlled or uncontrolled, should be discouraged. Crosskeying greatly reduces the keying capacity of a system, severely limits the security of the cross keyed cylin-der and makes the key system less flexible. Whenever possible, alternate solutions to cross keying shouldbe explored.If cross keying is required, each set's operating parameters need to be defined. The expressions can bewritten as follows: XAA1 oby AA1, AA2, AA, A (oby = operated by)X1X oby AC1 - AC5, AC10, AC, AIn the second example above, the “X” after the cylinder designation indicates that there is no change key(CK). It should be noted that cross keying requirements need to be defined at thetime bittings are being generated. Cross keyed cylinders are labeled with the “X” symbol.Additional DetailsThe letter "X" should not be used to designate a grand master. As stated above, this letter symbol isreserved for use in cross keying.The letters I and O should be avoided for use as grand master or master key symbols as they are some-times confused for numbers. To avoid confusion with O, Q should also be avoided.For systems with more than twenty-four master keys under a given grand, the counting numbers are usedbetween the letters of the pair. For example, A2A, A2B, A2C....A2Z are the symbols for master keys twenty-five through forty-eight.