The usual battle formation of a Roman legion(10 cohorts) was the acies triplex (triple line), 
 arranged as follows:

 1.prima acies(first line), consisting of 4 cohorts of experienced soldiers These were the first 
     to engage in battle

 2.secunda acies(second line), consisting of 3 cohorts, stationed about 150 feet behind the first
     line. Their duty was to relieve the wounded or fallen soldiers    of the first line.

 3.tertia acies(third line), consiting of the remaining 3 cohorts stationed father back. They
     were used as reserve abd as a defense aginst an attack from the rear.
   
   To give signals in battle, the Romans used the following:

   1.tuba(trumpet), a straight insturment about three feet long, made of metal.

   2.cornu(horn), a large curved insturment, also made of metal.