The g-shaped brass instrument used by the roman army for communicating
orders to troops is the

The G-shaped brass instrument used by the Roman army for communicating

orders to troops is the cornu.

Explanation:

The cornu is an instrument that is created with the evolution of the first horns used by the Etruscans. Its characteristic shape was almost circular with a crossbar to grab it that gave the structure greater strength. The cornu was widely used in the context of war since it was used to communicate orders during the battle, and also during the crossing of the troops marking the pace of march. It was also used in Roman military camps to emit signals, such as indicating the four vigils at night. When the cornu was heard in the camp for the first time it meant that they should prepare for the march. It was time to disassemble camping tents, collect luggage. At the second touch of cornu the baggage cars were loaded. When the touch sounded for the third time, it was the order to leave the camp. In general, the daily life of the legionaries was governed by the trumpet sounds (all the guards are ready to the sound of a lituus and are relieved to the sound of a cornu).

However, we also find the use of cornu in the civil sphere, mainly in games and funeral rituals. The use of cornu in the army extends well into the third century A.D.