By the centre? they haven't used that in donkeys. By the left or by the right is determined by the direction to which the squad/parade is marching, having turned to the left or right. When soldiers get into a squad in front of the drill sergeant, they start from the front/right and line up, three deep, towards the left, forming a squad. The first soldier who took the front/right position is the right-hand marker (RHM). Now, if the DS turns the squad to the left, the RHM is on the right, in relation to the squad, so the call is to march "by the right, quick march". If on the otherhand, the DS turns the squad to the right, the RHM is on the left, in relation to the rest of the squad, so the command is for them to march "By the left, quick march!"