The Pelham bit combines the function of a snaffle and the action of a curb bit
The top rein provides snaffle pressure to the tongue and corners of the mouth, while the bottom rein increases leverage and curb chain pressure
The Mullen mouth creates even pressure on the bars of the mouth and the horse's tongue
The Tom Thumb cheeks are shorter than on a traditional pelham making this a better bit for a less experienced rider due to the reduced curb action available