The SCREEN pennant is spoken, written, and transmitted SCREEN. It is used in signaling various screening situations. SUBSTITUTES Whenever possible, substitutes are used to prevent alphabet flags, numeral flags, or numeral pennants from appearing more than once in the same hoist. As their names imply, they are substitutes for other flags or pennants used in the hoist. FIRST SUB repeats the first flag or pennant in the hoist. SECOND SUB repeats the second flag or pennant in the hoist. THIRD SUB repeats the third flag or pennant in the hoist. FOURTH SUB repeats the fourth flag or pennant in the hoist. To illustrate, the signal CORPEN PORT ZERO ZERO ZERO would read CORPEN FORT ZERO 3rd 4th. THIRD SUB repeats the third flag, and FOURTH SUB repeats the fourth flag, which already repeats ZERO. When more than one halyard is used to hoist a signal, each hoist is considered separately as regards substitutes. When a tackline separates hoist components, it is disregarded in the substitute count. Substitutes also are used as “absence indicators” when a ship is not under way. This is discussed in chapter 10, “Honors and Ceremonies.” UNITS OF REFERENCE When a signal makes reference to numbers, distances, ranges, heights, depths, speeds, or weights, the unit of reference is as indicated (see fig. 5-7) unless otherwise stated in the meaning of the signal. However, for clarity, the units of reference are stated against some groups using the standard units, which otherwise would not need such a statement. SIGNALING TIMES AND DATES When the originator desires to have a signal executed at a specific time instead of when the signal is hauled down, the time indicator, TANGO flag, is used. In the text of signals, times are expressed as four numerals; the first two denote the hours (00-23) and Altitude .  .  .  .  .  .  .  .  .   hundreds of feet Distance   .  .  .  .  .  .  .  .  .   nautical miles (2,000 yards) Range   .  .  .  .  .  .  .  .  .  .   hundreds of yards Height   .  .  .  .  .  .  .  .  .  .   feet Depth .  .  .  .  .  .  .  .  .  .   feet speed    .  .  .  .  .  .  .  .  .  .   knots Weight  .  .  .  .  .  .  .  .  .  .   tons (2,000 pounds) Sector boundaries  .  .  .  .   tens of degrees Sector limits  .  .  .  .  .  .  .   thousands of yards Figure 5-7.—Reference units of measurement. the second two the minutes. ANSWER, instead of the last two figures of a time signal, may be used to express 30 minutes. Thus, 1630 is sent as 16 ANS. Date-time groups in the text of signals are expressed as six numerals plus the zone indicator; the first set of two numerals denotes the date, the second set the hour, and the third set the minutes. When unable to make this display in one hoist, you may break it between the date and the time group. When time is sent together with a signal group, TANGO has the following meanings: T preceding numerals—Action will commence at that time. T following numerals—Action will be completed by that time. T between numeral groups—Time by which action is to be completed and time at which action is to commence, respectively. CO2 means “Assume command.” Examples: CO2 TACK T1845—Commence assuming command at 1845. CO2 TACK 1845T—Complete assuming command by 1845. CO2 TACK 19T1845—Commence assuming command at 1845; complete assuming by 1900. When time is referred to in the meaning of the signal group, the flag indicator TANGO may be omitted, provided no confusion results. A time signal applies only to the group immediately preceding it. If signaled time applies to more than one group, flags BRAVO TANGO (BT) are 5-11