As above for rein selection.
However, there is a difference between a “gag” and an “elevator” or 3 ring. The true gag simply shifts the pressure of the reins to the corners of the mouth, OFF the bars. There is little poll pressure. Because elevators or 3 rings have leverage, they produce more poll pressure as well as the raising action of the bit in the mouth. IMO, your steering is better with a regular pulley gag with two reins than with an elevator. A pulley gag acts as a regular snaffle UNLESS the horse goes to dive down and tow on the bars of his mouth. If he does that, the action of the gag comes into play. The elevator or three ring is more in action ALL the time. Although the pulley gags seem to be “out of fashion” at this point in time, I like them way better.