There were some good answers already. To me the main reason is to not lose contact nor disturb the mouth when using the right hand for some of it's tasks. Like using the whip, which is one of the main reasons for putting the reins in the left hand when driving singles and pairs, when using the whip. But when driving team the main reason - besides the whip - is, when making a turn to take the loop in the leader rein one hand needs to move forward to take the loop. So when driving two handed, the moment that hand moves forward to take the loop, the contact on the rein in that hand is lost. Granted, this applies more to teams than to tandems, since teams are wider, thus the loop is more important. With a small and quickly reacting tandem, as Bridgette has it, leader loops are not as often necessary, and one can drive two handed without taking loops there mostly, but with a team that won't work. Also, of course I agree, that two handed is quicker and thus better for cones and hazards - and many World Class team drivers use it in dressage now too - but I still prefer Achenbach for training and for dressage for keeping the proper contact. For a quick turn in cones and hazards a quick drop of the contact is not such a big deal, when the horses are trained properly elsewise. For the comment what our ancestors did when plowing as well as what Amish do now, that's really a different story, as draft horses were not driven "on the bit" nor did they need to "bend" or go "round". That's were voice commands came from and are good. The farmer had his hands on the plow and the reins around his neck. So "gee" and "haw" were / are fine there. Hardy