In ancient Rome, a promagistrate (Latin: pro magistratu) was a person who was granted the power via prorogation to act in place of an ordinary magistrate in the field. This was normally pro consule or pro praetore, that is, in place of a consul or praetor, respectively. This was an expedient development, starting in 327 BC and becoming regular by 241 BC, that was meant to allow consuls and praetors to continue their activities in the field without disruption.
By allowing veteran commanders to stay in the field rather than being rotated out for someone who may not have had much experience in the theatre, the practice helped increase the chances of victory. Whether a commander, however, would be kept was largely decided politically and often motivated by commanders' ambitions. However, the effect of prorogation was to allow commanders to retain their positions as long as political support existed, weakening the republican check of the annual magistracy (and the rotation that implied) over commanders' activities.
