s been the practice in the Army:

In approaching or passing each other within saluting distance,
individuals or bodies of troops exchange salutes when at a distance of
about 6 paces. If they do not approach each other that closely, the
salute is exchanged at the point of nearest approach. For instance, if
the officer and soldier are approaching each other on the same
sidewalk, the hand is brought up to the headdress when about 6 paces
from the officer. If they are on opposite sides of the street, the
hand is brought up when about ten paces in advance of the officer. If
the officer and soldier are not going in opposite directions and the
officer does not approach within six paces, the salute is rendered
when the officer reaches the nearest point to the soldier. If a
soldier passes an officer from the rear, the hand is raised as he
reaches the officer; if an officer passes a soldier from the rear, the
soldier salutes just as the officer is about to pass him.

=1542. Officer entering room occupied by soldiers.= When an officer
enters a room where there are several enlisted men, the word
"attention" is given by someone who perceives him, when all rise,
uncover, and remain standing at attention until the officer leaves the
room or directs otherwise.

=1543. At meals.= Enlisted men at meals stop eating and remain seated
at attention when an officer enters the room.

=1544. When seated.= An enlisted man, if seated, rises on the approach
of an officer, faces toward him, stands at attention, and salutes.
Standing he faces an officer for the same purpose. If the parties
remain in the same place or on the same ground, such compliments need
not be repeated.

=1545. Soldier indoors.= Indoors, an unarmed enlisted man uncovers and
stands at attention upon the approach of an officer. If armed with
rifle, he renders the rifle salute at the order or trail.

(Note. According to custom, the term "indoors" is interpreted as
meaning military offices barracks, quarters and similar places,--it
does not mean such public places as stores, storehouses, riding halls,
stables, post exchange buildings, hotels, places of amusement, and
railway and steamboat stations. In such places an unarmed soldier
renders the right hand salute.)

=1546. Officers approaching number of soldiers in open.= When an
officer approaches a number of enlisted men out of doors, the word
"attention" should be given by someone who perceives him, when all
stand at atte