=189.= In all cases the sentinel must satisfy himself beyond a reasonable doubt that the parties are what they represent themselves to be and have a right to pass. If he is not satisfied, he must cause them to stand and call the corporal of the guard. So, likewise, if he have no authority to pass persons with the countersign, or when the party has not the countersign, or gives an incorrect one. =190.= A sentinel will not permit any person to approach so close as to prevent the proper use of his own weapon before recognizing the person or receiving the countersign. =191.= When two or more persons approach in one party, the sentinel, on receiving an answer that indicates that some one in the party has the countersign, will say, "=Advance one with the countersign=," and if the countersign is given correctly, will then say, "=Advance (so-and-so)=," repeating the answer to his challenge. Thus if the answer be "=Relief (friend with the countersign, patrol, etc.)=," the sentinel will say, "=Advance one with the countersign="; then "=Advance, relief (friends, patrol, etc.).=" =192.= If a person having the countersign approach alone, he is advanced to give the countersign. Thus if the answer be "=Friend with the countersign (or officer of the day, or etc.)=," the sentinel will say, "=Advance, friend (or officer of the day, or etc.) with the countersign="; then "=Advance, friend (or officer of the day, or etc.).=" =193.= If two or more persons approach a sentinel's post from different directions at the same time, all such persons are challenged in turn and required to halt and to remain halted until advanced. The senior is first advanced, in accordance with the foregoing rules. =194.= If a party is already advanced and in communication with a sentinel, the latter will challenge any other party that may approach; if the party challenged be senior to the one already on his post, the sentinel will advance the new party at once. The senior may allow him to advance any or all of the other parties; otherwise the sentinel will not advance any of them until the senior leaves him. He will then advance the senior only of the remaining parties, and so on. =195.= The following order of rank will govern a sentinel in advancing different persons or parties approaching his post: Commanding officers, officer of the day, officer of the guard, officers, patrols, reliefs, noncommissioned officers of the guard in order of rank, friends. =196.= A sentinel will never allow himself to be surprised, nor permit two parties to advance upon him at the same time. =197.= If no countersign be used, the rules for challenging are the same. The rules for advancing parties are modified only as follows: Instead of saying "=Advance (so-and-so) with the countersign=," the sentinel will say "=Advance (so-and-so) to be recognized.=" Upon recognition he will say, "=Advance (so-and-so.)="